feat(audio): per-track owner/credit + license (+ written to mp3 ID3 tags)
New per-track metadata: credit (copyright holder) + license. Editable in the
track editor (license with datalist presets: All rights reserved, CC BY/…/CC0).
- DB: audio_tracks.credit + .license.
- ID3: on upload and on every metadata edit the tags are written into the mp3
itself — copyright=credit, comment=license (new retagMp3() in the transcoder,
-c copy, no re-encode) → ownership travels with a download.
- Visible: "credit · license" line below each track (post-audio-track).
busters audio.css?v=7.
Co-Authored-By: Claude <noreply@…>