Address review feedback on cloud login

- time out status polling after 2 min so an abandoned browser flow
  doesn't leave the button stuck on "Waiting for browser…"
- handle non-JSON / non-object payloads from the exchange and account
  endpoints instead of 500ing after the state is consumed
- make singleton row creation race-safe (IntegrityError -> re-query)
- clear device_name on disconnect along with the rest of the metadata
- serve the dashboard URL from /cloud/status so the Manage link follows
  VOICEBOX_CLOUD_URL instead of hardcoding production
- keep a "Disconnecting…" label on the disconnect button while pending

backend/models.py

@@ -813,3 +813,4 @@ class CloudStatusResponse(BaseModel):
     account_user_id: Optional[str] = None
     key_prefix: Optional[str] = None
     connected_at: Optional[datetime] = None
+    dashboard_url: str