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/schemas/common.py

@@ -34,3 +34,22 @@ class NotificationListResponse(BaseModel):
 
 class UnreadCountResponse(BaseModel):
     count: int
+
+
+class NotificationPreferenceUpdate(BaseModel):
+    is_enabled: bool
+
+
+class NotificationTypeResponse(BaseModel):
+    type_key: str
+    plugin_name: str
+    category: str
+    label: str
+    description: str | None = None
+    is_enabled_by_default: bool
+    is_enabled: bool
+
+
+class NotificationPreferenceResponse(BaseModel):
+    type_key: str
+    is_enabled: bool