feat(phase-18): add in-app notification system (T-128, T-129)

backend/app/main.py

@@ -16,6 +16,7 @@ from app.routers import system as system_router
 from app.routers import metrics as metrics_router
 from app.routers import users as users_router
 from app.routers import audit as audit_router
+from app.routers import notifications as notifications_router
 from app.storage import ensure_bucket_exists
 from app.jobs.mitre_sync_job import start_scheduler, scheduler
 
@@ -56,6 +57,7 @@ app.include_router(system_router.router, prefix="/api/v1")
 app.include_router(metrics_router.router, prefix="/api/v1")
 app.include_router(users_router.router, prefix="/api/v1")
 app.include_router(audit_router.router, prefix="/api/v1")
+app.include_router(notifications_router.router, prefix="/api/v1")
 
 
 @app.get("/health")