Closes Scribe #614, #615, server half of #616 surfaced by the 2026-06-04 divergent-provider audit.
- streamURL helper now used everywhere /api/tracks/{id}/stream is built (was inline concat in playlists.go and cast_token.go); add streamURLWithExt for the .ext cast variant.
- audioContentType in media.go is the canonical file_format -> MIME lookup; mimeForFormat in cast_token.go is now a thin wrapper that overrides the unknown-format fallback to audio/mpeg (Sonos rejects octet-stream). Adds mpeg/vorbis/wave aliases. Subsonic's contentTypeForFormat stays frozen per docs.
- coverart.ResolveAlbumPath extracted; api and subsonic both delegate to it.
- ArtistRef gains SortName and CoverURL fields
- AlbumRef gains SortTitle field
- albumRefFrom now populates SortTitle from dbq.Album.SortTitle
- artistRefFrom now populates SortName from dbq.Artist.SortName
- New artistRefFromCovered helper builds CoverURL from nullable cover_album_id
- New HomePayload view type for GET /api/home response body
bvandeusen