feat: email notifications + deadline reminder system

Database:
- notification_preferences table (user_id, tenant_id, reminder days, email/digest toggles)
- notifications table (type, entity link, read/sent tracking, dedup index)

Backend:
- NotificationService with background goroutine checking reminders hourly
- CheckDeadlineReminders: finds deadlines due in N days per user prefs, creates notifications
- Overdue deadline detection and notification
- Daily digest at 8am: compiles pending notifications into one email
- SendEmail via `m mail send` CLI command
- Deduplication: same notification type + entity + day = skip
- API: GET/PATCH notifications, unread count, mark read/all-read
- API: GET/PUT notification-preferences with upsert

Frontend:
- NotificationBell in header with unread count badge (polls every 30s)
- Dropdown panel with notification list, type-colored dots, time-ago, entity links
- Mark individual/all as read
- NotificationSettings in Einstellungen page: reminder day toggles, email toggle, digest toggle

backend/cmd/server/main.go

@@ -36,7 +36,12 @@ func main() {
 	calDAVSvc.Start()
 	defer calDAVSvc.Stop()
 
-	handler := router.New(database, authMW, cfg, calDAVSvc)
+	// Start notification reminder service
+	notifSvc := services.NewNotificationService(database)
+	notifSvc.Start()
+	defer notifSvc.Stop()
+
+	handler := router.New(database, authMW, cfg, calDAVSvc, notifSvc)
 
 	slog.Info("starting KanzlAI API server", "port", cfg.Port)
 	if err := http.ListenAndServe(":"+cfg.Port, handler); err != nil {