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

apps/api/src/models/__init__.py

@@ -3,6 +3,7 @@ from src.models.config_profile import ConfigProfile, ConfigProfileInclude
 from src.models.git_repository import GitRepository
 from src.models.health_check import HealthCheck
 from src.models.instance_event import InstanceEvent
+from src.models.notification import Notification
 from src.models.project import Project
 from src.models.ssh_key import SSHKey
 from src.models.terminal_session import TerminalSessionModel
@@ -19,6 +20,7 @@ __all__ = [
     "GitRepository",
     "HealthCheck",
     "InstanceEvent",
+    "Notification",
     "Project",
     "SSHKey",
     "TerminalSessionModel",