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/core/notifications.py

@@ -6,10 +6,14 @@ import uuid
 from datetime import UTC
 from typing import Any
 
-from sqlalchemy import func, select, update
+from sqlalchemy import and_, func, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.models.notification import Notification
+from app.models.notification import (
+    Notification,
+    NotificationPreference,
+    NotificationType,
+)
 
 
 async def create_notification(
@@ -19,8 +23,37 @@ async def create_notification(
     type: str,
     title: str,
     body: str | None = None,
-) -> Notification:
-    """Create a new notification for a user."""
+) -> Notification | None:
+    """Create a new notification for a user if they have not disabled this type.
+
+    Returns None if the user has opted out of this notification type.
+    """
+    # Check user preference
+    pref = await db.execute(
+        select(NotificationPreference).where(
+            and_(
+                NotificationPreference.user_id == user_id,
+                NotificationPreference.type_key == type,
+                NotificationPreference.tenant_id == tenant_id,
+            )
+        )
+    )
+    pref_row = pref.scalar_one_or_none()
+
+    # If preference exists and is disabled, skip
+    if pref_row and not pref_row.is_enabled:
+        return None
+
+    # If no preference, check if type is enabled by default
+    if not pref_row:
+        type_def = await db.execute(
+            select(NotificationType).where(NotificationType.type_key == type)
+        )
+        type_row = type_def.scalar_one_or_none()
+        if type_row and not type_row.is_enabled_by_default:
+            return None
+
+    # Create notification
     notif = Notification(
         tenant_id=tenant_id,
         user_id=user_id,