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/notify_ack.go

@@ -70,7 +70,7 @@ func handleNotifyAck(db *sql.DB) http.HandlerFunc {
 			// already acknowledged (e.g. from the web UI, or an earlier tap of
 			// the same button) — either way nothing the responder did wrong, so
 			// report the actual state rather than assuming "resolved", and let
-			// ntfy show a success toast rather than a failure.
+			// the provider show a success toast rather than a failure.
 			inc, err := fetchIncident(r.Context(), db, incidentID)
 			if err != nil {
 				serverError(w, r, err)