feat(connect): return site_numeric_id in heartbeat response

Local backend needs the cloud DB integer PK to call Connect API
endpoints (menu sync, order pending poll). Heartbeat is the only
authenticated channel available at startup, so we piggyback the id
there rather than adding a new endpoint.

Changes:
  - schemas/site.py: site_numeric_id: int | None added to HeartbeatResponse
  - routers/heartbeat.py: site_numeric_id=site.id included in response

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

cloud_backend/routers/heartbeat.py

@@ -39,4 +39,5 @@ def heartbeat(
         expires_at=site.license_expires_at,
         latest_version=settings.LATEST_VERSION,
         waiter_domain=site.waiter_domain,
+        site_numeric_id=site.id,
     )

cloud_backend/schemas/site.py

@@ -57,3 +57,4 @@ class HeartbeatResponse(BaseModel):
     expires_at: datetime
     latest_version: str | None = None
     waiter_domain: str | None = None
+    site_numeric_id: int | None = None  # cloud DB pk — needed by Connect sync loops