Configure Mux auto-generated captions for on-demand video assets

Steps

1. When creating a Mux asset via POST to '/video/v1/assets', include a 'generated_subtitles' array in the request body with at least one object specifying 'language_code' (e.g., 'en') and optionally 'name' and 'passthrough' fields
2. To add auto-generated captions to an existing asset, use POST to '/video/v1/assets/{ASSET_ID}/tracks' with 'type' set to 'subtitles', 'language_code' set to your target language, and 'closed_captions' set to true
3. Set 'language_code' to 'auto' to enable automatic language detection — Mux uses OpenAI Whisper under the hood and supports 22 languages for on-demand video
4. Poll the asset's track status via GET '/video/v1/assets/{ASSET_ID}' and check that the track's 'status' field is 'ready' before attempting to retrieve or serve the captions
5. For live streams, include 'generated_subtitles' in the POST to '/video/v1/live-streams' — live captions are delivered in near real time and the first 6000 minutes per month are included at no additional charge
6. Retrieve the caption track as a VTT file or request a plain-text transcript via GET to '/video/v1/assets/{ASSET_ID}/tracks/{TRACK_ID}/transcript' once the track status is ready