- 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>
