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/helpers.go

@@ -63,6 +63,13 @@ func isUniqueViolation(err error) bool {
 	return errors.As(err, &pgErr) && pgErr.Code == pgerrcode.UniqueViolation
 }
 
+// isForeignKeyViolation reports whether err is a reference to a row that does not
+// exist, which callers turn into 404 rather than 500.
+func isForeignKeyViolation(err error) bool {
+	var pgErr *pgconn.PgError
+	return errors.As(err, &pgErr) && pgErr.Code == pgerrcode.ForeignKeyViolation
+}
+
 func respond(w http.ResponseWriter, status int, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)