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/i18n/locales/en.json

@@ -407,7 +407,7 @@
     "testConnection": "Test Connection",
     "connectionSuccess": "Connection successful.",
     "syncNow": "Sync Now",
-    "syncSuccess": "{{count}} emails synced.",
+    "syncSuccess": "Sync successful",
     "selectAccount": "Select Account",
     "selectAccountFirst": "Please select an account first.",
     "folders": "Folders",
@@ -560,9 +560,19 @@
     "sortSubject": "Subject",
     "sortAsc": "Ascending",
     "sortDesc": "Descending",
-    "syncSuccess": "Sync successful",
     "syncFailed": "Sync failed",
     "syncing": "Syncing...",
     "autoSyncEnabled": "Auto-sync enabled"
+  },
+  "notifications": {
+    "title": "Notification Settings",
+    "description": "Choose which notifications you want to receive",
+    "enabled": "Enabled",
+    "disabled": "Disabled",
+    "saved": "Preference saved",
+    "category": {
+      "mail": "Mail",
+      "general": "General"
+    }
   }
 }