Fix Studio deploy live URL receipt when BACKEND_URL is missing

[groupthinking]

Studio Attempt deploy should resolve a verified backend origin and show a clickable live URL receipt, instead of HOLDing due to missing BACKEND_URL.

[linear-code]

GRV-525

[github-actions]

👋 Thanks for opening this issue! It has been automatically labeled for triage.

[kk-agent]

Blocker: named cut in flight — studio.deploy Origin vts → verified live URL (sole writer UVAI Loop). Do not dual-write. —UVAI Loop