The album download tagged album_artist with the request's `artist`, which can be a
single track's performer — the lead single "TAKEDOWN" is by TWICE, so a whole
KPop-Demon-Hunters soundtrack got album_artist=TWICE on every track. Derive the album
artist from the iTunes tracks' collectionArtistName (the album-level artist, identical
across the album) and fall back to the request artist only if iTunes omits it. This is
the documented intent (mainArtist = collectionArtistName when available) and keeps
album_artist consistent so Navidrome doesn't split the album.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>