Bug 1: Editing any AD cue never updated descriptive_transcript.txt in GCS.
Bug 2A: Uploading replacement CC or AD .vtt had the same root cause.
After saving captions or AD VTT, read the other stream from GCS if not
provided in the request, merge both via generate_descriptive_transcript(),
upload the result to {job_id}/{lang}/descriptive_transcript.txt, and
update lang_output["descriptive_transcript_gcs"] before the DB write.
Bug 2B (CC-only job → 400 on empty audio_description_vtt): already fixed
by the existing `if request.audio_description_vtt:` guard (empty string
and None are both falsy) and frontend `adVtt || undefined` sending no
field rather than an empty string.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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