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/main.py

@@ -21,9 +21,11 @@ from src.api.tool_definitions import router as tool_definitions_router
 from src.api.tool_instances import router as tool_instances_router
 from src.api.tool_instances import sessions_router
 from src.api.tool_types import router as tool_types_router
+from src.api.notifications import router as notifications_router
 from src.api.user_config import router as user_config_router
 from src.api.users import router as users_router
 from src.config import Settings
+from src.models.notification import Notification  # noqa: F401 – Alembic model discovery
 from src.models.terminal_session import TerminalSessionModel  # noqa: F401 – Alembic model discovery
 from src.database import init_database
 from src.logging_config import (
@@ -156,4 +158,5 @@ app.include_router(sessions_router)
 app.include_router(instance_proxy_router)
 app.include_router(terminal_router)
 app.include_router(events_router)
+app.include_router(notifications_router)
 app.mount("/uploads", StaticFiles(directory="uploads"), name="uploads")