Sign in through an OpenID Connect provider, and from a terminal

terdut can now sign people in through any OIDC provider (written against
Authentik), and let groups at the provider decide who may sign in, which
teams they belong to and whether they administer the install. Password
login keeps working alongside it; TERDUT_PASSWORD_LOGIN=false turns it off,
and is refused at startup unless SSO is configured. With no TERDUT_OIDC_*
setting nothing changes, so every existing install behaves as before.

Identity is (issuer, subject), never email or username: those are mutable
at the provider and a recycled address must not inherit an account. An
existing user is linked by email only when the provider marks it verified,
or TERDUT_OIDC_TRUST_EMAIL is set, which Authentik needs.

Group grants are marked source='oidc' on team_members and users, and the
sync changes only those rows. Hand-made memberships and administrators
are left alone, and the sync bypasses the last-owner and last-admin guards
because the provider is the source of truth for what it grants. Editing
managed access by hand is refused with 409, since the next sign-in would
undo it. The web UI badges it as SSO and disables the controls.

Groups are read only at sign-in, so an SSO session carries a hard ceiling
(sessions.max_expires_at, 12h by default) that sliding never extends.
There is no refresh token, which means API keys of somebody removed at the
provider stay valid until an administrator disables the user. That is
accepted and documented, not fixed.

A client with no browser, the TUI over SSH, signs in with a device code
run by terdut itself (POST /api/oidc/device and /device/token), so the
terminal never talks to the provider and ends up with the ordinary
terdut_session cookie. Only a browser session can approve a code; an API
key cannot. /device?code= sends a signed-out visitor through sign-in and
back, which is what oidc_logins.next is for.

oauth2 is pinned to v0.36.0: v0.37 needs Go 1.26 and the Dockerfile
builds on 1.25.

Migrations 011 and 012 add tables and defaulted columns only.

internal/api/users.go

@@ -96,7 +96,7 @@ func handleBootstrap(db *sql.DB) http.HandlerFunc {
 func handleListUsers(db *sql.DB) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		rows, err := db.QueryContext(r.Context(),
-			"SELECT id, username, email, created_at, ntfy_topic, is_admin, disabled_at FROM users ORDER BY id")
+			"SELECT id, username, email, created_at, ntfy_topic, is_admin, admin_source, disabled_at FROM users ORDER BY id")
 		if err != nil {
 			respond(w, http.StatusInternalServerError, errResp("internal error"))
 			return
@@ -108,7 +108,7 @@ func handleListUsers(db *sql.DB) http.HandlerFunc {
 			var u models.User
 			var ts int64
 			var disabled *int64
-			if err := rows.Scan(&u.ID, &u.Username, &u.Email, &ts, &u.NtfyTopic, &u.IsAdmin, &disabled); err != nil {
+			if err := rows.Scan(&u.ID, &u.Username, &u.Email, &ts, &u.NtfyTopic, &u.IsAdmin, &u.AdminSource, &disabled); err != nil {
 				respond(w, http.StatusInternalServerError, errResp("internal error"))
 				return
 			}
@@ -329,8 +329,8 @@ func fetchUser(ctx context.Context, db *sql.DB, id int64) (models.User, error) {
 	var ts int64
 	var disabled *int64
 	err := db.QueryRowContext(ctx,
-		"SELECT id, username, email, created_at, ntfy_topic, is_admin, disabled_at FROM users WHERE id = $1", id).
-		Scan(&u.ID, &u.Username, &u.Email, &ts, &u.NtfyTopic, &u.IsAdmin, &disabled)
+		"SELECT id, username, email, created_at, ntfy_topic, is_admin, admin_source, disabled_at FROM users WHERE id = $1", id).
+		Scan(&u.ID, &u.Username, &u.Email, &ts, &u.NtfyTopic, &u.IsAdmin, &u.AdminSource, &disabled)
 	if err != nil {
 		return u, err
 	}
@@ -361,6 +361,18 @@ func handleSetAdmin(db *sql.DB) http.HandlerFunc {
 		}
 
 		if !*req.IsAdmin {
+			var managed bool
+			if err := db.QueryRowContext(r.Context(),
+				"SELECT EXISTS (SELECT 1 FROM users WHERE id = $1 AND is_admin AND admin_source = 'oidc')",
+				id).Scan(&managed); err != nil {
+				respond(w, http.StatusInternalServerError, errResp("internal error"))
+				return
+			}
+			if managed {
+				respond(w, http.StatusConflict, errResp("administrator access is managed by single sign-on; change the user's groups in the identity provider"))
+				return
+			}
+
 			caller, _ := userFromContext(r.Context())
 			if caller.ID == id {
 				respond(w, http.StatusConflict, errResp("cannot revoke your own administrator access"))