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/api/hooks.ts

@@ -459,6 +459,42 @@ export function useDeleteNotification() {
   });
 }
 
+export interface NotificationTypeItem {
+  type_key: string;
+  plugin_name: string;
+  category: string;
+  label: string;
+  description: string | null;
+  is_enabled_by_default: boolean;
+  is_enabled: boolean;
+}
+
+export function useNotificationTypes() {
+  return useQuery({
+    queryKey: ['notification-types'],
+    queryFn: () => apiGet<{ items: NotificationTypeItem[] }>('/notifications/types'),
+  });
+}
+
+export function useNotificationPreferences() {
+  return useQuery({
+    queryKey: ['notification-preferences'],
+    queryFn: () => apiGet<{ items: { type_key: string; is_enabled: boolean }[] }>('/notifications/preferences'),
+  });
+}
+
+export function useUpdateNotificationPreference() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: ({ typeKey, isEnabled }: { typeKey: string; isEnabled: boolean }) =>
+      apiPatch(`/notifications/preferences/${typeKey}`, { is_enabled: isEnabled }),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['notification-preferences'] });
+      queryClient.invalidateQueries({ queryKey: ['notification-types'] });
+    },
+  });
+}
+
 export function usePlugins() {
   return useQuery({
     queryKey: ['plugins'],