Add self-service sign-up and invite links

First half of #7. Until now the only way to get an account was for
somebody who already had one to create it, and the login page told people
to "ask an admin" -- workable for one operator, impossible for a team.

Two modes, chosen by an administrator in the settings table: invite_only,
which is the default, and open. A third domain-restricted mode was
considered and dropped, because with no email in this server there is
nothing to verify an address against and it would only check the domain
of a string somebody typed.

The default is the closed door. An install that gets a public hostname
before anybody has thought about sign-up should not be collecting
accounts from the internet, and the failure mode of a typo in the setting
is invite_only rather than open.

An invite is a link, not an email. Adding SMTP to send one message would
be a subsystem to run, secure and monitor; the person inviting sends the
link however they already talk to the person they are inviting. A link
carries the team and the role, because an account in no team sees an
empty queue and can be paged by nobody -- that is not a state to invite
somebody into. Links are single-use by default, expire after seven days,
and can be revoked before that: a link that works forever is a credential
nobody remembers issuing, sitting in a chat log.

The uses counter is incremented inside the sign-up transaction and
guarded by `uses < max_uses`, so two people redeeming the last use at
once cannot both get in.

GET /api/signup reports the mode and whether a link is usable, so the
form can say "this link has expired" before somebody picks a password
rather than after. It gives one answer for expired, revoked, used up and
never existed: telling a stranger which it was tells them something about
links they do not hold.

Sign-up signs you in. The alternative is a form that says "now go and log
in", which is the same credential typed twice. login and signup now share
startSession rather than each minting a cookie.

Rate-limited per address on its own limiter, not login's: a burst of
sign-ups must not lock somebody out of logging in.

The settings table grew a second shape for this. It held only durations;
signup_mode is a word from a fixed list, so the admin endpoint now
validates everything before writing anything -- a request that sets two
settings and gets one wrong changes neither.

Still to come in #7: the sign-up and invite-redemption pages, the
first-run checklist, and the in-app integration instructions. The schema
carries onboarding_dismissed_at for the checklist already.

Claude-Session: https://claude.ai/code/session_01RHPj4ggeFdEjKKfm4SHbD7

internal/api/auth.go

@@ -139,6 +139,34 @@ func hashPassword(pw string) (string, error) {
 	return string(h), err
 }
 
+// startSession mints a session and sets the cookie. Shared by login and
+// sign-up: somebody who has just chosen a password is signed in, rather than
+// being sent to a form to type the same credential again.
+func startSession(w http.ResponseWriter, r *http.Request, db *sql.DB, userID int64, publicURL string) error {
+	raw, tokenHash, err := randomToken()
+	if err != nil {
+		return err
+	}
+	now := time.Now()
+	if _, err := db.ExecContext(r.Context(), `
+		INSERT INTO sessions (token_hash, user_id, created_at, last_seen_at, expires_at, user_agent)
+		VALUES ($1, $2, $3, $4, $5, $6)`,
+		tokenHash, userID, now.Unix(), now.Unix(), now.Add(sessionTTL).Unix(), r.UserAgent()); err != nil {
+		return err
+	}
+
+	http.SetCookie(w, &http.Cookie{
+		Name:     sessionCookie,
+		Value:    raw,
+		Path:     "/",
+		MaxAge:   int(sessionTTL.Seconds()),
+		HttpOnly: true,
+		Secure:   cookieSecure(publicURL, r),
+		SameSite: http.SameSiteLaxMode,
+	})
+	return nil
+}
+
 // handleLogin exchanges a username and password for a session cookie.
 func handleLogin(db *sql.DB, limiter *loginLimiter, publicURL string) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
@@ -182,29 +210,10 @@ func handleLogin(db *sql.DB, limiter *loginLimiter, publicURL string) http.Handl
 		}
 		limiter.clear(userKey)
 
-		raw, tokenHash, err := randomToken()
-		if err != nil {
+		if err := startSession(w, r, db, userID, publicURL); err != nil {
 			respond(w, http.StatusInternalServerError, errResp("internal error"))
 			return
 		}
-		now := time.Now()
-		if _, err := db.ExecContext(r.Context(), `
-			INSERT INTO sessions (token_hash, user_id, created_at, last_seen_at, expires_at, user_agent)
-			VALUES ($1, $2, $3, $4, $5, $6)`,
-			tokenHash, userID, now.Unix(), now.Unix(), now.Add(sessionTTL).Unix(), r.UserAgent()); err != nil {
-			respond(w, http.StatusInternalServerError, errResp("internal error"))
-			return
-		}
-
-		http.SetCookie(w, &http.Cookie{
-			Name:     sessionCookie,
-			Value:    raw,
-			Path:     "/",
-			MaxAge:   int(sessionTTL.Seconds()),
-			HttpOnly: true,
-			Secure:   cookieSecure(publicURL, r),
-			SameSite: http.SameSiteLaxMode,
-		})
 
 		user, err := fetchUser(r.Context(), db, userID)
 		if err != nil {

internal/api/router.go

@@ -15,6 +15,12 @@ import (
 // notify disables notifications. Dead man's switches are per team and read from
 // the database, so nothing about them is wired in here.
 func NewRouter(db *sql.DB, notify NotifyConfig, cfg config.Config) http.Handler {
+	// One limiter each, both process-wide for the life of the router: login
+	// counts failed passwords, sign-up counts account creation, and mixing the
+	// two would let a burst of sign-ups lock somebody out of logging in.
+	loginLimit := newLoginLimiter()
+	signupLimiter := newLoginLimiter()
+
 	r := chi.NewRouter()
 	r.Use(middleware.Logger)
 	r.Use(middleware.Recoverer)
@@ -39,9 +45,16 @@ func NewRouter(db *sql.DB, notify NotifyConfig, cfg config.Config) http.Handler
 	// JSON 404 every unknown /api path gets.
 	r.Post("/api/integrations/{key}/alertmanager", handleIntegrationWebhook(db, notify))
 
+	// Signing up. Both are unauthenticated by necessity: the caller has no
+	// account yet. The info endpoint says whether the door is open and whether
+	// an invite link is good, so the form can say so before somebody picks a
+	// password.
+	r.Get("/api/signup", handleSignupInfo(db))
+	r.Post("/api/signup", handleSignup(db, signupLimiter, notify.PublicURL))
+
 	// Signing in to the web UI. Login trades a password for a session cookie,
 	// which AuthMiddleware accepts in place of an API key.
-	r.Post("/api/login", handleLogin(db, newLoginLimiter(), notify.PublicURL))
+	r.Post("/api/login", handleLogin(db, loginLimit, notify.PublicURL))
 	r.Post("/api/logout", handleLogout(db, notify.PublicURL))
 
 	// All other /api routes require a valid API key.
@@ -109,6 +122,11 @@ func NewRouter(db *sql.DB, notify NotifyConfig, cfg config.Config) http.Handler
 		r.Post("/api/teams/{teamID}/members", handleAddTeamMember(db))
 		r.Delete("/api/teams/{teamID}/members/{userID}", handleRemoveTeamMember(db))
 
+		// Invite links into this team.
+		r.Get("/api/teams/{teamID}/invites", handleListInvites(db))
+		r.Post("/api/teams/{teamID}/invites", handleCreateInvite(db, notify.PublicURL))
+		r.Delete("/api/teams/{teamID}/invites/{inviteID}", handleRevokeInvite(db))
+
 		// A team's escalation ladder: who is paged when nobody answers.
 		r.Get("/api/teams/{teamID}/escalation", handleGetEscalation(db))
 		r.Put("/api/teams/{teamID}/escalation", handleSetEscalation(db))

internal/api/settings.go

@@ -90,6 +90,16 @@ func SeedSettings(ctx context.Context, db *sql.DB, cfg config.Config) error {
 type settingsResponse struct {
 	Editable map[string]settingValue `json:"editable"`
 	FromEnv  map[string]string       `json:"from_env"`
+
+	// Choices are settings that are a word from a fixed list rather than a
+	// duration. One so far: who may create an account.
+	Choices map[string]choiceValue `json:"choices"`
+}
+
+type choiceValue struct {
+	Value       string   `json:"value"`
+	Options     []string `json:"options"`
+	Description string   `json:"description"`
 }
 
 type settingValue struct {
@@ -104,6 +114,14 @@ func handleGetSettings(db *sql.DB, cfg config.Config) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		out := settingsResponse{
 			Editable: map[string]settingValue{},
+			Choices: map[string]choiceValue{
+				SettingSignupMode: {
+					Value:   signupMode(r.Context(), db),
+					Options: []string{SignupInviteOnly, SignupOpen},
+					Description: "who may create an account: invite_only means a link from a team owner, " +
+						"open means anybody who can reach this server",
+				},
+			},
 			FromEnv: map[string]string{
 				// Never the ntfy token or the DSN: both are credentials, and an
 				// admin page that renders them turns a browser tab into a place
@@ -137,7 +155,7 @@ func handleGetSettings(db *sql.DB, cfg config.Config) http.HandlerFunc {
 // sit in the table looking like configuration and doing nothing.
 func handleSetSettings(db *sql.DB) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		var req map[string]int64
+		var req map[string]any
 		if err := decodeJSON(r, &req); err != nil {
 			respond(w, http.StatusBadRequest, errResp("invalid request body"))
 			return
@@ -147,17 +165,37 @@ func handleSetSettings(db *sql.DB) http.HandlerFunc {
 			return
 		}
 
-		for key, secs := range req {
-			b, known := settingBounds[key]
-			if !known {
-				respond(w, http.StatusBadRequest, errResp("unknown setting: "+key))
-				return
-			}
-			d := time.Duration(secs) * time.Second
-			if d < b.min || d > b.max {
-				respond(w, http.StatusBadRequest, errResp(
-					key+" must be between "+b.min.String()+" and "+b.max.String()))
-				return
+		// Validate everything before writing anything: a request that sets two
+		// settings and gets one wrong should change neither.
+		values := map[string]string{}
+		for key, raw := range req {
+			switch key {
+			case SettingSignupMode:
+				mode, _ := raw.(string)
+				if mode != SignupOpen && mode != SignupInviteOnly {
+					respond(w, http.StatusBadRequest,
+						errResp("signup_mode must be "+SignupInviteOnly+" or "+SignupOpen))
+					return
+				}
+				values[key] = mode
+			default:
+				b, known := settingBounds[key]
+				if !known {
+					respond(w, http.StatusBadRequest, errResp("unknown setting: "+key))
+					return
+				}
+				secs, ok := raw.(float64) // JSON numbers decode as float64
+				if !ok {
+					respond(w, http.StatusBadRequest, errResp(key+" must be a number of seconds"))
+					return
+				}
+				d := time.Duration(int64(secs)) * time.Second
+				if d < b.min || d > b.max {
+					respond(w, http.StatusBadRequest, errResp(
+						key+" must be between "+b.min.String()+" and "+b.max.String()))
+					return
+				}
+				values[key] = strconv.FormatInt(int64(secs), 10)
 			}
 		}
 
@@ -168,13 +206,13 @@ func handleSetSettings(db *sql.DB) http.HandlerFunc {
 		}
 		defer tx.Rollback() //nolint:errcheck
 
-		for key, secs := range req {
+		for key, value := range values {
 			if _, err := tx.ExecContext(r.Context(), `
 				INSERT INTO settings (key, value, updated_at)
 				VALUES ($1, $2, `+nowEpoch+`)
 				ON CONFLICT (key) DO UPDATE SET
 					value = excluded.value, updated_at = excluded.updated_at`,
-				key, strconv.FormatInt(secs, 10)); err != nil {
+				key, value); err != nil {
 				respond(w, http.StatusInternalServerError, errResp("internal error"))
 				return
 			}

internal/api/signup.go

@@ -0,0 +1,412 @@
+package api
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"net/http"
+	"strconv"
+	"strings"
+	"time"
+
+	"git.ryuvia.com/niklas/terdut-server/internal/models"
+	"github.com/go-chi/chi/v5"
+)
+
+// SettingSignupMode says who may create an account. It lives in the settings
+// table with the other behaviour settings, so an administrator changes it in
+// the admin page rather than in a chart.
+//
+// Two modes, not three. A domain-restricted mode was considered and dropped:
+// with no email in this server there is nothing to verify an address against,
+// so it would check the domain of a string somebody typed — a speed bump
+// dressed as a control.
+const (
+	SettingSignupMode = "signup_mode"
+
+	SignupInviteOnly = "invite_only"
+	SignupOpen       = "open"
+)
+
+// defaultSignupMode is invite-only. An install that gets a public hostname
+// before anybody has thought about sign-up should not be collecting accounts
+// from the internet by default.
+const defaultSignupMode = SignupInviteOnly
+
+// inviteTTL is how long a new invite link lives. Long enough to send it and be
+// read tomorrow, short enough that a link in an old chat log stops working.
+const inviteTTL = 7 * 24 * time.Hour
+
+// signupMode reads the current mode, falling back to invite-only for a missing
+// or unrecognised value: the failure mode of a typo in this setting should be
+// the closed door, not the open one.
+func signupMode(ctx context.Context, db *sql.DB) string {
+	var raw string
+	if err := db.QueryRowContext(ctx,
+		"SELECT value FROM settings WHERE key = $1", SettingSignupMode).Scan(&raw); err != nil {
+		return defaultSignupMode
+	}
+	if raw != SignupOpen && raw != SignupInviteOnly {
+		return defaultSignupMode
+	}
+	return raw
+}
+
+// handleSignupInfo tells the sign-up page what it may offer, without requiring
+// a session: whether open sign-up is on, and whether the invite in the URL is
+// any good. A bad invite is better reported before somebody picks a password.
+func handleSignupInfo(db *sql.DB) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		out := map[string]any{"mode": signupMode(r.Context(), db)}
+
+		if token := r.URL.Query().Get("invite"); token != "" {
+			inv, err := loadInvite(r.Context(), db, token)
+			switch {
+			case err == nil:
+				out["invite_valid"] = true
+				out["invite_team"] = inv.teamName
+			default:
+				// Deliberately one answer for expired, revoked, used up and
+				// never existed. Telling a stranger which it was tells them
+				// something about links they do not hold.
+				out["invite_valid"] = false
+			}
+		}
+		respond(w, http.StatusOK, out)
+	}
+}
+
+type invite struct {
+	id       int64
+	teamID   int64
+	teamName string
+	role     string
+}
+
+// loadInvite resolves a raw token to a usable invite, or an error. Usable means
+// it exists, has not been revoked, has not expired and has uses left.
+func loadInvite(ctx context.Context, q querier, token string) (invite, error) {
+	var inv invite
+	err := q.QueryRowContext(ctx, `
+		SELECT i.id, i.team_id, t.name, i.role
+		FROM   invites i
+		JOIN   teams t ON t.id = i.team_id
+		WHERE  i.token_hash = $1
+		  AND  i.revoked_at IS NULL
+		  AND  i.expires_at > `+nowEpoch+`
+		  AND  i.uses < i.max_uses`, hashToken(token)).
+		Scan(&inv.id, &inv.teamID, &inv.teamName, &inv.role)
+	if errors.Is(err, sql.ErrNoRows) {
+		return invite{}, errInviteUnusable
+	}
+	return inv, err
+}
+
+var errInviteUnusable = errors.New("invite is not usable")
+
+// handleSignup creates an account, and puts it somewhere.
+//
+// Rate-limited on the same limiter as login, by address: sign-up is the other
+// unauthenticated endpoint that writes, and an open install without this is a
+// way to fill somebody's user table.
+func handleSignup(db *sql.DB, limiter *loginLimiter, publicURL string) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		addr := clientAddr(r)
+		if limiter.blocked("signup:"+addr, maxSignupsPerAddr) {
+			respond(w, http.StatusTooManyRequests, errResp("too many sign-ups from this address"))
+			return
+		}
+
+		var req struct {
+			Username string `json:"username"`
+			Email    string `json:"email"`
+			Password string `json:"password"`
+			Invite   string `json:"invite"`
+			TeamName string `json:"team_name"`
+		}
+		if err := decodeJSON(r, &req); err != nil {
+			respond(w, http.StatusBadRequest, errResp("invalid request body"))
+			return
+		}
+		req.Username = strings.TrimSpace(req.Username)
+		req.Email = strings.TrimSpace(req.Email)
+		req.TeamName = strings.TrimSpace(req.TeamName)
+
+		if req.Username == "" || req.Email == "" {
+			respond(w, http.StatusBadRequest, errResp("username and email are required"))
+			return
+		}
+		if msg := validatePassword(req.Password); msg != "" {
+			respond(w, http.StatusBadRequest, errResp(msg))
+			return
+		}
+
+		mode := signupMode(r.Context(), db)
+		var inv invite
+		hasInvite := false
+		if req.Invite != "" {
+			var err error
+			inv, err = loadInvite(r.Context(), db, req.Invite)
+			if err != nil {
+				limiter.fail("signup:" + addr)
+				respond(w, http.StatusForbidden, errResp("this invite link is not usable"))
+				return
+			}
+			hasInvite = true
+		}
+		if !hasInvite && mode != SignupOpen {
+			// No invite and the door is shut. Not 404: the endpoint exists and
+			// saying so is how somebody knows to ask for a link.
+			respond(w, http.StatusForbidden,
+				errResp("sign-up is invite-only on this server"))
+			return
+		}
+		if !hasInvite && req.TeamName == "" {
+			// Open sign-up with no team would create an account that sees an
+			// empty queue and can be paged by nobody.
+			respond(w, http.StatusBadRequest, errResp("team_name is required"))
+			return
+		}
+
+		hash, err := hashPassword(req.Password)
+		if err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+
+		tx, err := db.BeginTx(r.Context(), nil)
+		if err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+		defer tx.Rollback() //nolint:errcheck
+
+		var userID int64
+		var invitedVia *int64
+		if hasInvite {
+			invitedVia = &inv.id
+		}
+		if err := tx.QueryRowContext(r.Context(), `
+			INSERT INTO users (username, email, password_hash, invited_via)
+			VALUES ($1, $2, $3, $4) RETURNING id`,
+			req.Username, req.Email, hash, invitedVia).Scan(&userID); err != nil {
+			if isUniqueViolation(err) {
+				respond(w, http.StatusConflict, errResp("username or email already exists"))
+				return
+			}
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+
+		teamID, role := inv.teamID, inv.role
+		if !hasInvite {
+			// Open sign-up makes a team, and its creator owns it.
+			if err := tx.QueryRowContext(r.Context(),
+				"INSERT INTO teams (name) VALUES ($1) RETURNING id", req.TeamName).Scan(&teamID); err != nil {
+				if isUniqueViolation(err) {
+					respond(w, http.StatusConflict, errResp("a team with that name already exists"))
+					return
+				}
+				respond(w, http.StatusInternalServerError, errResp("internal error"))
+				return
+			}
+			role = models.RoleOwner
+		}
+
+		if _, err := tx.ExecContext(r.Context(),
+			"INSERT INTO team_members (team_id, user_id, role) VALUES ($1, $2, $3)",
+			teamID, userID, role); err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+
+		if hasInvite {
+			// Counted inside the transaction, so two people redeeming the last
+			// use of a link at once cannot both get in.
+			res, err := tx.ExecContext(r.Context(),
+				"UPDATE invites SET uses = uses + 1 WHERE id = $1 AND uses < max_uses", inv.id)
+			if err != nil {
+				respond(w, http.StatusInternalServerError, errResp("internal error"))
+				return
+			}
+			if n, _ := res.RowsAffected(); n == 0 {
+				respond(w, http.StatusForbidden, errResp("this invite link is not usable"))
+				return
+			}
+		}
+
+		if err := tx.Commit(); err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+
+		// Signed in immediately: the alternative is a form that says "now go
+		// and log in", which is the same credential typed twice.
+		if err := startSession(w, r, db, userID, publicURL); err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+		user, _ := fetchUser(r.Context(), db, userID)
+		respond(w, http.StatusCreated, meResponse{User: user, HasPassword: true})
+	}
+}
+
+// maxSignupsPerAddr is looser than the login limit: several people joining from
+// one office share an address, and the thing being limited is account creation
+// rather than password guessing.
+const maxSignupsPerAddr = 10
+
+// ---------------------------------------------------------------------------
+// Invites
+// ---------------------------------------------------------------------------
+
+type inviteJSON struct {
+	ID        int64     `json:"id"`
+	TeamID    int64     `json:"team_id"`
+	Role      string    `json:"role"`
+	CreatedAt time.Time `json:"created_at"`
+	ExpiresAt time.Time `json:"expires_at"`
+	MaxUses   int64     `json:"max_uses"`
+	Uses      int64     `json:"uses"`
+	Revoked   bool      `json:"revoked"`
+
+	// URL is the whole link, returned once when the invite is created. Like an
+	// integration key, only its hash is stored.
+	URL string `json:"url,omitempty"`
+}
+
+func handleListInvites(db *sql.DB) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		teamID, ok := teamParam(w, r)
+		if !ok {
+			return
+		}
+		if !requireTeamOwner(w, r, teamID) {
+			return
+		}
+
+		rows, err := db.QueryContext(r.Context(), `
+			SELECT id, team_id, role, created_at, expires_at, max_uses, uses, revoked_at
+			FROM   invites
+			WHERE  team_id = $1
+			ORDER  BY id DESC`, teamID)
+		if err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+		defer rows.Close()
+
+		out := []inviteJSON{}
+		for rows.Next() {
+			var i inviteJSON
+			var created, expires int64
+			var revoked *int64
+			if err := rows.Scan(&i.ID, &i.TeamID, &i.Role, &created, &expires,
+				&i.MaxUses, &i.Uses, &revoked); err != nil {
+				respond(w, http.StatusInternalServerError, errResp("internal error"))
+				return
+			}
+			i.CreatedAt = time.Unix(created, 0).UTC()
+			i.ExpiresAt = time.Unix(expires, 0).UTC()
+			i.Revoked = revoked != nil
+			out = append(out, i)
+		}
+		if err := rows.Err(); err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+		respond(w, http.StatusOK, out)
+	}
+}
+
+// handleCreateInvite mints a link into this team. Owner-only, like the rest of
+// a team's configuration: deciding who joins is configuring the team.
+func handleCreateInvite(db *sql.DB, publicURL string) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		teamID, ok := teamParam(w, r)
+		if !ok {
+			return
+		}
+		if !requireTeamOwner(w, r, teamID) {
+			return
+		}
+
+		var req struct {
+			Role    string `json:"role"`
+			MaxUses int64  `json:"max_uses"`
+		}
+		if err := decodeJSON(r, &req); err != nil {
+			respond(w, http.StatusBadRequest, errResp("invalid request body"))
+			return
+		}
+		if req.Role == "" {
+			req.Role = models.RoleMember
+		}
+		if req.Role != models.RoleOwner && req.Role != models.RoleMember {
+			respond(w, http.StatusBadRequest, errResp("role must be owner or member"))
+			return
+		}
+		if req.MaxUses == 0 {
+			req.MaxUses = 1
+		}
+		if req.MaxUses < 1 || req.MaxUses > 100 {
+			respond(w, http.StatusBadRequest, errResp("max_uses must be between 1 and 100"))
+			return
+		}
+
+		raw, hash, err := randomToken()
+		if err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+		caller, _ := userFromContext(r.Context())
+		expires := time.Now().Add(inviteTTL)
+
+		var out inviteJSON
+		var created, expiresAt int64
+		if err := db.QueryRowContext(r.Context(), `
+			INSERT INTO invites (token_hash, team_id, role, created_by, expires_at, max_uses)
+			VALUES ($1, $2, $3, $4, $5, $6)
+			RETURNING id, team_id, role, created_at, expires_at, max_uses, uses`,
+			hash, teamID, req.Role, caller.ID, expires.Unix(), req.MaxUses).
+			Scan(&out.ID, &out.TeamID, &out.Role, &created, &expiresAt, &out.MaxUses, &out.Uses); err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+		out.CreatedAt = time.Unix(created, 0).UTC()
+		out.ExpiresAt = time.Unix(expiresAt, 0).UTC()
+		out.URL = strings.TrimSuffix(publicURL, "/") + "/signup?invite=" + raw
+		respond(w, http.StatusCreated, out)
+	}
+}
+
+// handleRevokeInvite stops a link working without waiting for it to expire.
+func handleRevokeInvite(db *sql.DB) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		teamID, ok := teamParam(w, r)
+		if !ok {
+			return
+		}
+		if !requireTeamOwner(w, r, teamID) {
+			return
+		}
+		id, err := strconv.ParseInt(chi.URLParam(r, "inviteID"), 10, 64)
+		if err != nil {
+			respond(w, http.StatusBadRequest, errResp("invalid invite id"))
+			return
+		}
+
+		res, err := db.ExecContext(r.Context(),
+			"UPDATE invites SET revoked_at = "+nowEpoch+
+				" WHERE id = $1 AND team_id = $2 AND revoked_at IS NULL", id, teamID)
+		if err != nil {
+			respond(w, http.StatusInternalServerError, errResp("internal error"))
+			return
+		}
+		if n, _ := res.RowsAffected(); n == 0 {
+			respond(w, http.StatusNotFound, errResp("not found"))
+			return
+		}
+		w.WriteHeader(http.StatusNoContent)
+	}
+}

internal/api/signup_test.go

@@ -0,0 +1,270 @@
+package api_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/cookiejar"
+	"testing"
+)
+
+// signup posts to the unauthenticated sign-up endpoint, the way the form does,
+// and returns the response and a client holding whatever cookie came back.
+func signup(t *testing.T, s *ts, body map[string]any) (*http.Response, *http.Client) {
+	t.Helper()
+	data, _ := json.Marshal(body)
+	jar, _ := cookiejar.New(nil)
+	client := &http.Client{Jar: jar}
+	req, _ := http.NewRequest(http.MethodPost, s.URL+"/api/signup", bytes.NewReader(data))
+	req.Header.Set("Content-Type", "application/json")
+	resp, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("signup: %v", err)
+	}
+	return resp, client
+}
+
+// invite mints a link into the default team and returns its raw token.
+func invite(t *testing.T, s *ts, role string, maxUses int64) string {
+	t.Helper()
+	var out struct {
+		URL string `json:"url"`
+	}
+	decode(t, s.req(t, http.MethodPost, "/api/teams/"+defaultTeam+"/invites",
+		map[string]any{"role": role, "max_uses": maxUses}), &out)
+	if out.URL == "" {
+		t.Fatal("no invite URL returned")
+	}
+	// ...?invite=<token>
+	i := len(out.URL) - 1
+	for ; i >= 0 && out.URL[i] != '='; i-- {
+	}
+	return out.URL[i+1:]
+}
+
+func setSignupMode(t *testing.T, s *ts, mode string) {
+	t.Helper()
+	resp := s.req(t, http.MethodPut, "/api/admin/settings", map[string]any{"signup_mode": mode})
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusNoContent {
+		t.Fatalf("set signup mode: %d", resp.StatusCode)
+	}
+}
+
+// The default is the closed door. An install that gets a public hostname before
+// anybody has thought about sign-up should not be collecting accounts.
+func TestSignup_InviteOnlyByDefault(t *testing.T) {
+	s := newTS(t)
+
+	var info map[string]any
+	decode(t, s.req(t, http.MethodGet, "/api/signup", nil), &info)
+	if info["mode"] != "invite_only" {
+		t.Errorf("default sign-up mode is %v, want invite_only", info["mode"])
+	}
+
+	resp, _ := signup(t, s, map[string]any{
+		"username": "stranger", "email": "s@test.com", "password": "correct-horse-battery",
+	})
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Errorf("sign-up without an invite: expected 403, got %d", resp.StatusCode)
+	}
+}
+
+// An invite carries the team and the role, so redeeming one lands somewhere
+// usable rather than in an account that sees an empty queue.
+func TestSignup_InviteCreatesAMemberOfThatTeam(t *testing.T) {
+	s := newTS(t)
+	token := invite(t, s, "member", 1)
+
+	// The form checks the link before asking for a password.
+	var info map[string]any
+	decode(t, s.req(t, http.MethodGet, "/api/signup?invite="+token, nil), &info)
+	if info["invite_valid"] != true {
+		t.Fatalf("a fresh invite should be valid: %v", info)
+	}
+	if info["invite_team"] != "Default" {
+		t.Errorf("the form should name the team: %v", info["invite_team"])
+	}
+
+	resp, client := signup(t, s, map[string]any{
+		"username": "newcomer", "email": "n@test.com",
+		"password": "correct-horse-battery", "invite": token,
+	})
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("redeeming an invite: %d", resp.StatusCode)
+	}
+	var me struct {
+		User struct {
+			ID      int64 `json:"id"`
+			IsAdmin bool  `json:"is_admin"`
+		} `json:"user"`
+	}
+	decode(t, resp, &me)
+	if me.User.IsAdmin {
+		t.Error("somebody who signs up must not be an administrator")
+	}
+
+	// Signed in already: the cookie came back with the response.
+	got, err := client.Get(s.URL + "/api/teams")
+	if err != nil {
+		t.Fatal(err)
+	}
+	teams := list(t, got)
+	if len(teams) != 1 || teams[0]["name"] != "Default" || teams[0]["role"] != "member" {
+		t.Errorf("expected membership of Default as member, got %v", teams)
+	}
+}
+
+// A single-use link is single-use, and the check is inside the transaction so
+// two people redeeming the last use at once cannot both get in.
+func TestSignup_InviteCannotBeUsedTwice(t *testing.T) {
+	s := newTS(t)
+	token := invite(t, s, "member", 1)
+
+	first, _ := signup(t, s, map[string]any{
+		"username": "first", "email": "f@test.com",
+		"password": "correct-horse-battery", "invite": token,
+	})
+	first.Body.Close()
+	if first.StatusCode != http.StatusCreated {
+		t.Fatalf("first redemption: %d", first.StatusCode)
+	}
+
+	second, _ := signup(t, s, map[string]any{
+		"username": "second", "email": "s@test.com",
+		"password": "correct-horse-battery", "invite": token,
+	})
+	second.Body.Close()
+	if second.StatusCode != http.StatusForbidden {
+		t.Errorf("second redemption: expected 403, got %d", second.StatusCode)
+	}
+
+	// And the link reports itself unusable before anybody types a password.
+	var info map[string]any
+	decode(t, s.req(t, http.MethodGet, "/api/signup?invite="+token, nil), &info)
+	if info["invite_valid"] != false {
+		t.Error("a used-up invite should report itself invalid")
+	}
+}
+
+// Revoking stops a link without waiting for it to expire.
+func TestSignup_RevokedInviteStopsWorking(t *testing.T) {
+	s := newTS(t)
+	token := invite(t, s, "member", 5)
+
+	invites := list(t, s.req(t, http.MethodGet, "/api/teams/"+defaultTeam+"/invites", nil))
+	if len(invites) != 1 {
+		t.Fatalf("expected one invite, got %d", len(invites))
+	}
+	id := int64(invites[0]["id"].(float64))
+
+	resp := s.req(t, http.MethodDelete, "/api/teams/"+defaultTeam+"/invites/"+id64(id), nil)
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusNoContent {
+		t.Fatalf("revoke: %d", resp.StatusCode)
+	}
+
+	used, _ := signup(t, s, map[string]any{
+		"username": "late", "email": "l@test.com",
+		"password": "correct-horse-battery", "invite": token,
+	})
+	used.Body.Close()
+	if used.StatusCode != http.StatusForbidden {
+		t.Errorf("a revoked invite: expected 403, got %d", used.StatusCode)
+	}
+}
+
+// Open sign-up makes a team, because an account in no team sees an empty queue
+// and can be paged by nobody.
+func TestSignup_OpenModeMakesATeam(t *testing.T) {
+	s := newTS(t)
+	setSignupMode(t, s, "open")
+
+	missing, _ := signup(t, s, map[string]any{
+		"username": "solo", "email": "s@test.com", "password": "correct-horse-battery",
+	})
+	missing.Body.Close()
+	if missing.StatusCode != http.StatusBadRequest {
+		t.Errorf("open sign-up with no team name: expected 400, got %d", missing.StatusCode)
+	}
+
+	resp, client := signup(t, s, map[string]any{
+		"username": "solo", "email": "s@test.com",
+		"password": "correct-horse-battery", "team_name": "Solo",
+	})
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("open sign-up: %d", resp.StatusCode)
+	}
+
+	got, err := client.Get(s.URL + "/api/teams")
+	if err != nil {
+		t.Fatal(err)
+	}
+	teams := list(t, got)
+	if len(teams) != 1 || teams[0]["name"] != "Solo" || teams[0]["role"] != "owner" {
+		t.Errorf("the creator should own their new team, got %v", teams)
+	}
+}
+
+// Switching the mode is an administrator's decision, and it takes effect at
+// once rather than at the next restart.
+func TestSignup_ModeIsAnAdminSetting(t *testing.T) {
+	s := newTS(t)
+	_, call := member(t, s, "plain")
+
+	resp := call(http.MethodPut, "/api/admin/settings", map[string]any{"signup_mode": "open"})
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Errorf("a member changing the mode: expected 403, got %d", resp.StatusCode)
+	}
+
+	bad := s.req(t, http.MethodPut, "/api/admin/settings", map[string]any{"signup_mode": "everybody"})
+	bad.Body.Close()
+	if bad.StatusCode != http.StatusBadRequest {
+		t.Errorf("an unknown mode: expected 400, got %d", bad.StatusCode)
+	}
+
+	setSignupMode(t, s, "open")
+	var info map[string]any
+	decode(t, s.req(t, http.MethodGet, "/api/signup", nil), &info)
+	if info["mode"] != "open" {
+		t.Errorf("the change should be visible at once, got %v", info["mode"])
+	}
+}
+
+// Minting a link is configuring the team, so it is an owner's job.
+func TestSignup_InvitesAreOwnerOnly(t *testing.T) {
+	s := newTS(t)
+	_, call := member(t, s, "plain")
+
+	resp := call(http.MethodPost, "/api/teams/"+defaultTeam+"/invites", map[string]any{"role": "member"})
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Errorf("a member minting an invite: expected 403, got %d", resp.StatusCode)
+	}
+}
+
+// A password still has to be a password, and a taken username is still taken.
+func TestSignup_ValidatesLikeTheRestOfTheServer(t *testing.T) {
+	s := newTS(t)
+	token := invite(t, s, "member", 5)
+
+	short, _ := signup(t, s, map[string]any{
+		"username": "shorty", "email": "sh@test.com", "password": "abc", "invite": token,
+	})
+	short.Body.Close()
+	if short.StatusCode != http.StatusBadRequest {
+		t.Errorf("a short password: expected 400, got %d", short.StatusCode)
+	}
+
+	taken, _ := signup(t, s, map[string]any{
+		"username": "admin", "email": "other@test.com",
+		"password": "correct-horse-battery", "invite": token,
+	})
+	taken.Body.Close()
+	if taken.StatusCode != http.StatusConflict {
+		t.Errorf("an existing username: expected 409, got %d", taken.StatusCode)
+	}
+}

internal/db/migrations/007_signup_invites.sql

@@ -0,0 +1,49 @@
+-- Self-service sign-up, and the invite links that make it useful.
+--
+-- Until now the only way to get an account was for somebody who already had one
+-- to create it, and the login page told people to "ask an admin". That is a
+-- workable arrangement for one operator and an impossible one for a team.
+--
+-- An invite is a link, not an email: this server has no SMTP and adding it to
+-- send one message would be a new subsystem to run, secure and monitor. The
+-- person inviting sends the link however they already talk to the person they
+-- are inviting.
+CREATE TABLE invites (
+    id BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
+
+    -- SHA-256 of the raw token, like api_keys, the integration keys and the
+    -- acknowledgement tokens. A leaked database hands nobody an account.
+    token_hash TEXT NOT NULL UNIQUE,
+
+    -- Which team the invitee lands in, and as what. An invite always names a
+    -- team: an account in no team sees an empty queue and can be paged by
+    -- nobody, which is not a state to invite somebody into.
+    team_id BIGINT NOT NULL REFERENCES teams(id) ON DELETE CASCADE,
+    role    TEXT   NOT NULL CHECK (role IN ('owner', 'member')),
+
+    created_by BIGINT REFERENCES users(id) ON DELETE SET NULL,
+    created_at BIGINT NOT NULL DEFAULT FLOOR(EXTRACT(EPOCH FROM now()))::bigint,
+
+    -- Invites expire. A link that works forever is a credential nobody
+    -- remembers issuing, sitting in a chat log.
+    expires_at BIGINT NOT NULL,
+
+    -- Single-use by default: max_uses 1. A team onboarding six people at once
+    -- can raise it rather than minting six links.
+    max_uses BIGINT NOT NULL DEFAULT 1 CHECK (max_uses > 0 AND max_uses <= 100),
+    uses     BIGINT NOT NULL DEFAULT 0,
+
+    -- Revoked by hand, separately from expiry, so "this link is no longer
+    -- wanted" and "this link timed out" stay distinguishable in the listing.
+    revoked_at BIGINT
+);
+
+CREATE INDEX invites_team_idx ON invites(team_id);
+
+-- Who redeemed which invite. Kept after the invite is gone — the answer to "how
+-- did this account get here" should outlive the link that made it.
+ALTER TABLE users ADD COLUMN invited_via BIGINT REFERENCES invites(id) ON DELETE SET NULL;
+
+-- Where a person is in the first-run checklist, so it can be resumed and
+-- dismissed rather than nagging forever. One row per user, created on demand.
+ALTER TABLE users ADD COLUMN onboarding_dismissed_at BIGINT;