Videos (all phone-compatible .mp4, plus common containers) that live in an
artist or album folder are now indexed alongside audio:
- Move 'mp4' out of AUDIO_EXT into a new VIDEO_EXT (mp4/m4v/mkv/mov/webm/avi) —
it was wrongly treated as an audio track before.
- ffprobeVideo captures file/title/durationSec/width/height per video.
- meta.json gains an optional `videos: IndexVideo[]`; a folder with only videos
now still gets a meta.json. Manifest entries gain optional `videos: N`.
- Video files join the album version signature (changes bump `v` for resync).
- New `videosIndexed` status counter + resync-log line.
Location is inherent in the folder rel (always an artist/album dir), so no
extra location field is needed. MUSIC_API.md documents the videos field +
manifest count. No poster/thumbnail generation yet (folder cover is reused).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>