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/schemas/device.py

@@ -90,6 +90,8 @@ class DeviceResponse(BaseModel):
     groups: list[DeviceGroupRef] = []
     site_id: Optional[uuid.UUID] = None
     site_name: Optional[str] = None
+    sector_id: Optional[uuid.UUID] = None
+    sector_name: Optional[str] = None
     created_at: datetime
 
     model_config = {"from_attributes": True}