Add notification channels and a provider abstraction

Users get a notification_channels table (replacing users.ntfy_topic); pages fan
out to every enabled channel through a notify.Provider registry, with ntfy as
the first provider. Fallbacks are typed {type, address}. Schema edited in place:
existing databases must be recreated.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

internal/api/teams.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"git.ryuvia.com/niklas/terdut-server/internal/models"
+	"git.ryuvia.com/niklas/terdut-server/internal/notify"
 	"github.com/go-chi/chi/v5"
 )
 
@@ -283,8 +284,9 @@ type memberStatus struct {
 	// NextShift is the first day after today the rota has them (YYYY-MM-DD).
 	NextShift *string `json:"next_shift,omitempty"`
 
-	// Pageable is whether they have an ntfy topic and an enabled account — the
-	// conditions pageLevel and the notifier skip on. Never the topic itself.
+	// Pageable is whether they have an enabled channel of a type this server can
+	// deliver, and an enabled account — the conditions pageLevel and the notifier
+	// skip on. Never the channel's address.
 	Pageable bool   `json:"pageable"`
 	Problem  string `json:"problem,omitempty"`
 
@@ -294,7 +296,7 @@ type memberStatus struct {
 
 // handleListTeamMembers names everybody in a team, with their status. Visible to
 // any member: you can see who else is on the rota you are on.
-func handleListTeamMembers(db *sql.DB) http.HandlerFunc {
+func handleListTeamMembers(db *sql.DB, providers *notify.Registry) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		teamID, ok := teamParam(w, r)
 		if !ok {
@@ -306,7 +308,8 @@ func handleListTeamMembers(db *sql.DB) http.HandlerFunc {
 
 		rows, err := db.QueryContext(r.Context(), `
 			SELECT m.team_id, m.user_id, u.username, m.role, m.joined_at, m.source,
-			       u.ntfy_topic IS NOT NULL AND u.ntfy_topic <> '',
+			       EXISTS (SELECT 1 FROM notification_channels c
+			                WHERE c.user_id = u.id AND c.enabled AND c.type = ANY($3::text[])),
 			       u.disabled_at IS NOT NULL,
 			       GREATEST(
 			         COALESCE((SELECT MAX(last_seen_at) FROM sessions WHERE user_id = u.id), 0),
@@ -318,7 +321,7 @@ func handleListTeamMembers(db *sql.DB) http.HandlerFunc {
 			FROM   team_members m
 			JOIN   users u ON u.id = m.user_id
 			WHERE  m.team_id = $1
-			ORDER  BY u.username`, teamID, todayUTC())
+			ORDER  BY u.username`, teamID, todayUTC(), providerTypes(providers))
 		if err != nil {
 			serverError(w, r, err)
 			return
@@ -329,9 +332,9 @@ func handleListTeamMembers(db *sql.DB) http.HandlerFunc {
 		for rows.Next() {
 			var m memberStatus
 			var joined, lastActive int64
-			var hasTopic, disabled bool
+			var hasChannel, disabled bool
 			if err := rows.Scan(&m.TeamID, &m.UserID, &m.Username, &m.Role, &joined, &m.Source,
-				&hasTopic, &disabled, &lastActive, &m.OnCall, &m.NextShift); err != nil {
+				&hasChannel, &disabled, &lastActive, &m.OnCall, &m.NextShift); err != nil {
 				serverError(w, r, err)
 				return
 			}
@@ -343,8 +346,8 @@ func handleListTeamMembers(db *sql.DB) http.HandlerFunc {
 			switch {
 			case disabled:
 				m.Problem = "account is disabled"
-			case !hasTopic:
-				m.Problem = "has no ntfy topic"
+			case !hasChannel:
+				m.Problem = "has no notification channel"
 			}
 			m.Pageable = m.Problem == ""
 			switch {