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

cloud_backend/models/admin.py

@@ -0,0 +1,11 @@
+from sqlalchemy import Column, Integer, String
+from database import Base
+
+
+class Admin(Base):
+    __tablename__ = "admins"
+
+    id = Column(Integer, primary_key=True, index=True)
+    username = Column(String, unique=True, nullable=False)
+    password_hash = Column(String, nullable=False)
+    role = Column(String, default="sysadmin")