feat(11-01): create sites table migration, model, and schemas

- Add migration 030 with sites table, RLS policy, and device site_id FK
- Add Site SQLAlchemy model with tenant isolation
- Add site_id nullable FK and relationship to Device model
- Add sites relationship to Tenant model
- Register Site in models __init__.py
- Add SiteCreate, SiteUpdate, SiteResponse, SiteListResponse schemas

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

backend/app/models/__init__.py

@@ -13,6 +13,7 @@ from app.models.device import (
 from app.models.alert import AlertRule, NotificationChannel, AlertRuleChannel, AlertEvent
 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.audit_log import AuditLog
 from app.models.maintenance_window import MaintenanceWindow
 from app.models.api_key import ApiKey
@@ -28,6 +29,7 @@ __all__ = [
     "DeviceGroupMembership",
     "DeviceTagAssignment",
     "DeviceStatus",
+    "Site",
     "AlertRule",
     "NotificationChannel",
     "AlertRuleChannel",