feat(14-01): add sector CRUD backend with migration, model, service, and router

- Create sectors table migration (034) with RLS and devices.sector_id FK
- Add Sector ORM model with site_id and tenant_id foreign keys
- Add SectorCreate/Update/Response/ListResponse Pydantic schemas
- Implement sector_service with CRUD and device assignment functions
- Add sectors router with GET/POST/PUT/DELETE and device sector assignment
- Register sectors router in main.py
- Add sector_id and sector_name to Device model and DeviceResponse

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -434,6 +434,7 @@ def create_app() -> FastAPI:
     from app.routers.winbox_remote import router as winbox_remote_router
     from app.routers.sites import router as sites_router
     from app.routers.links import router as links_router
+    from app.routers.sectors import router as sectors_router
 
     app.include_router(auth_router, prefix="/api")
     app.include_router(tenants_router, prefix="/api")
@@ -465,6 +466,7 @@ def create_app() -> FastAPI:
     app.include_router(winbox_remote_router, prefix="/api")
     app.include_router(sites_router, prefix="/api")
     app.include_router(links_router, prefix="/api")
+    app.include_router(sectors_router, prefix="/api")
 
     # Health check endpoints
     @app.get("/health", tags=["health"])