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/models/__init__.py

@@ -8,7 +8,7 @@ from app.models.auth import ApiToken, PasswordResetToken
 from app.models.company import Company
 from app.models.contact import CompanyContact, Contact
 from app.models.currency import Currency
-from app.models.notification import Notification
+from app.models.notification import Notification, NotificationPreference, NotificationType
 from app.models.plugin import Plugin, PluginMigration
 from app.models.role import Role
 from app.models.sequence import Sequence
@@ -28,6 +28,8 @@ __all__ = [
     "AuditLog",
     "DeletionLog",
     "Notification",
+    "NotificationType",
+    "NotificationPreference",
     "PasswordResetToken",
     "ApiToken",
     "Company",