feat: notification preferences system with plugin registry

- Add NotificationType and NotificationPreference models
- Add get_notification_types() to BasePlugin for plugin registration
- Add sync_notification_types() to PluginRegistry
- Update create_notification() to check user preferences (returns Notification|None)
- Add API endpoints: GET /types, GET /preferences, PATCH /preferences/{type_key}
- Add NotificationPreferenceUpdate schema
- Mail plugin registers 10 notification types with metadata
- Add Alembic migration 0017 for new tables + seed data
- Frontend: SettingsNotifications page with toggle switches
- Frontend: Settings tab, route, hooks for notification preferences
- i18n: notification settings keys (de/en)

frontend/src/routes/index.tsx

@@ -29,6 +29,7 @@ import { DmsPage } from '@/pages/Dms';
 import { DmsTrashPage } from '@/pages/DmsTrash';
 import { MailPage } from '@/pages/Mail';
 import { MailSettingsPage } from '@/pages/MailSettings';
+import { SettingsNotificationsPage } from '@/pages/SettingsNotifications';
 
 const router = createBrowserRouter([
   {
@@ -81,6 +82,7 @@ const router = createBrowserRouter([
           { path: 'taxes', element: <SettingsTaxesPage /> },
           { path: 'sequences', element: <SettingsSequencesPage /> },
           { path: 'mail', element: <MailSettingsPage /> },
+          { path: 'notifications', element: <SettingsNotificationsPage /> },
         ],
       },
     ],