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/models/__init__.py

@@ -14,6 +14,7 @@ from app.models.alert import AlertRule, NotificationChannel, AlertRuleChannel, A
 from app.models.firmware import FirmwareVersion, FirmwareUpgradeJob
 from app.models.config_template import ConfigTemplate, ConfigTemplateTag, TemplatePushJob
 from app.models.site import Site
+from app.models.sector import Sector
 from app.models.audit_log import AuditLog
 from app.models.maintenance_window import MaintenanceWindow
 from app.models.api_key import ApiKey
@@ -32,6 +33,7 @@ __all__ = [
     "DeviceTagAssignment",
     "DeviceStatus",
     "Site",
+    "Sector",
     "AlertRule",
     "NotificationChannel",
     "AlertRuleChannel",