feat: waiter domain + local IP tracking

- Site model: add waiter_domain and last_seen_local_ip columns
- HeartbeatRequest: accept optional local_ip field from local backend
- HeartbeatResponse: return waiter_domain to local backend
- heartbeat router: persist local_ip on each check-in
- SiteDetailPage: show Public IP / Local IP separately, add Waiter Domain
  card with inline edit modal

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cloud_backend/models/site.py

@@ -19,3 +19,5 @@ class Site(Base):
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     last_seen_at = Column(DateTime(timezone=True), nullable=True)
     last_seen_ip = Column(String, nullable=True)
+    last_seen_local_ip = Column(String, nullable=True)
+    waiter_domain = Column(String, nullable=True)