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)

app/plugins/base.py

@@ -72,6 +72,16 @@ class BasePlugin(ABC):
         """
         return None
 
+    # ─── Notification Types ───
+
+    def get_notification_types(self) -> list[dict[str, Any]]:
+        """Return notification types this plugin registers.
+
+        Override in subclasses to register notification types.
+        Each dict must have: type_key, category, label, description, is_enabled_by_default
+        """
+        return []
+
     # ─── Route Registration ───
 
     def get_routes(self) -> list[APIRouter]: