Backend:
- VttContentResponse gets etag field (SHA1 of captions+AD content)
- VttUpdateRequest gets if_match field (optional)
- GET /jobs/{id}/vtt: computes and returns etag
- PATCH /jobs/{id}/vtt: if if_match present, fetches current content, recomputes
hash, returns 409 Conflict if mismatch
Frontend:
- VttContentResponse type + VttUpdateRequest type updated
- QCDetail stores vttEtag from GET response
- All updateVttMutation calls pass if_match: vttEtag
- 409 responses show specific "Conflict: another user has modified" message
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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