feat: notification center backend core (PR-1)

- Add notifications table with Alembic migration
- Notification model with user-scoped indexing and partial index on unread
- NotificationService singleton with create/list/count/mark-read/dismiss
- FastAPI router: GET /notifications, GET /unread, PATCH /{id}/read,
  POST /mark-all-read, DELETE /{id}
- Mute categories filtering from UserConfig
- 13 unit tests for NotificationService
- 10 integration tests for API endpoints
- Updated test_models.py with new table registration

Quality gates: pytest 23 new passed, ruff clean

openspec/changes/notification-center/.openspec.yaml

@@ -0,0 +1,5 @@
+name: notification-center
+description: Modular centralized notification management with UI notification center
+owner: Gentle AI
+created_at: 2026-05-28
+status: in-progress