feat(11-01): create site service, router, and wire into app

- Add site_service with CRUD, health rollup, device assignment functions
- Add sites router with 8 endpoints (CRUD + assign/unassign/bulk-assign)
- RBAC: viewer for reads, operator for writes, tenant_admin for delete
- Wire sites_router into main.py with /api prefix
- Health rollup computes device_count, online_count, online_percent per site

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

backend/app/main.py

@@ -390,6 +390,7 @@ def create_app() -> FastAPI:
     from app.routers.settings import router as settings_router
     from app.routers.remote_access import router as remote_access_router
     from app.routers.winbox_remote import router as winbox_remote_router
+    from app.routers.sites import router as sites_router
 
     app.include_router(auth_router, prefix="/api")
     app.include_router(tenants_router, prefix="/api")
@@ -419,6 +420,7 @@ def create_app() -> FastAPI:
     app.include_router(settings_router, prefix="/api")
     app.include_router(remote_access_router, prefix="/api")
     app.include_router(winbox_remote_router, prefix="/api")
+    app.include_router(sites_router, prefix="/api")
 
     # Health check endpoints
     @app.get("/health", tags=["health"])