feat: integrated audio player + animated transcript follower

- Video detail (done videos): added 'Download audio' button; removed the
  manual 'Thumbnail' button (thumbnails auto-download now, so it was redundant).
- New 'GET /api/videos/{id}/audio' endpoint (also serves HEAD for probing),
  serving data/audio/<video_id>.mp3. Audio job outtmpl switched to %(id)s so
  files are addressable per video.
- Integrated <audio> player appears in the detail view once the MP3 is present
  (probed via HEAD on openVideo; polled after a download job until ready).
- Synced transcript follower: as audio plays, the matching segment is
  highlighted (seg-active) and auto-scrolled into view like a karaoke/lyrics
  tracker. Clicking any segment timestamp or chapter seeks the audio to that
  point (falls back to scroll when no audio). Playback indicator pulses while
  playing.

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