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/internal/router/router.go

@@ -15,7 +15,7 @@ import (
 	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
 )
 
-func New(db *sqlx.DB, authMW *auth.Middleware, cfg *config.Config, calDAVSvc *services.CalDAVService) http.Handler {
+func New(db *sqlx.DB, authMW *auth.Middleware, cfg *config.Config, calDAVSvc *services.CalDAVService, notifSvc *services.NotificationService) http.Handler {
 	mux := http.NewServeMux()
 
 	// Services
@@ -43,6 +43,12 @@ func New(db *sqlx.DB, authMW *auth.Middleware, cfg *config.Config, calDAVSvc *se
 	noteSvc := services.NewNoteService(db)
 	dashboardSvc := services.NewDashboardService(db)
 
+	// Notification handler (optional — nil in tests)
+	var notifH *handlers.NotificationHandler
+	if notifSvc != nil {
+		notifH = handlers.NewNotificationHandler(notifSvc, db)
+	}
+
 	// Handlers
 	tenantH := handlers.NewTenantHandler(tenantSvc)
 	caseH := handlers.NewCaseHandler(caseSvc)
@@ -137,6 +143,16 @@ func New(db *sqlx.DB, authMW *auth.Middleware, cfg *config.Config, calDAVSvc *se
 		scoped.HandleFunc("POST /api/ai/summarize-case", aiLimiter.LimitFunc(aiH.SummarizeCase))
 	}
 
+	// Notifications
+	if notifH != nil {
+		scoped.HandleFunc("GET /api/notifications", notifH.List)
+		scoped.HandleFunc("GET /api/notifications/unread-count", notifH.UnreadCount)
+		scoped.HandleFunc("PATCH /api/notifications/{id}/read", notifH.MarkRead)
+		scoped.HandleFunc("PATCH /api/notifications/read-all", notifH.MarkAllRead)
+		scoped.HandleFunc("GET /api/notification-preferences", notifH.GetPreferences)
+		scoped.HandleFunc("PUT /api/notification-preferences", notifH.UpdatePreferences)
+	}
+
 	// CalDAV sync endpoints
 	if calDAVSvc != nil {
 		calDAVH := handlers.NewCalDAVHandler(calDAVSvc)