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/pages/Settings.tsx

@@ -15,6 +15,7 @@ export function SettingsPage() {
     { to: '/settings/taxes', label: t('taxes.title'), icon: '\ud83d\udccb' },
     { to: '/settings/sequences', label: t('sequences.title'), icon: '\ud83d\udd22' },
     { to: '/settings/mail', label: t('mail.settings'), icon: '\ud83d\udce7' },
+    { to: '/settings/notifications', label: t('settings.notifications'), icon: '\ud83d\udd14' },
   ];
 
   return (