Phase 4: cloud backend — licensing, heartbeat, site management

- New cloud_backend/ FastAPI service on port 8001 (SQLite for dev, swappable to PostgreSQL)
- Endpoints: sysadmin auth (JWT), site registration, lock/unlock, heartbeat (X-Site-ID + X-Site-Key headers)
- Default sysadmin seeded on first startup from ADMIN_USERNAME/ADMIN_PASSWORD env vars
- cloud_backend added to docker-compose.yml with persistent data volume at ./data/cloud/
- local_backend cloud_sync.py updated to use correct /api/heartbeat/ endpoint with header auth
- local_backend config.py: added SITE_KEY setting
- Smoke tested: login, register site, heartbeat, lock, unlock, list all pass

docker-compose.yml

@@ -1,4 +1,14 @@
 services:
+  cloud_backend:
+    build: ./cloud_backend
+    ports:
+      - "8001:8001"
+    restart: unless-stopped
+    env_file:
+      - ./cloud_backend/.env
+    volumes:
+      - ./data/cloud:/app/data
+
   backend:
     build: ./local_backend
     ports: