Jellyfin's listSongs streamed media files via GetCursor, which never
hydrates artwork, so songs emitted entity-id image tags and no blurhash.
media_file now uses the same id pre-pass as the other three cursors
(album/artist/playlist), for consistency, but on a separate method,
GetCursorWithArtwork: GetCursor itself must stay untouched, since it's
also the scanner's hot path and the scanner never reads artwork.
Measured on 1,000,000 tracks, the pre-pass over all ids costs +41.8 MB
heap and +298 ms versus GetCursor's bounded +0.0 MB. The Jellyfin path
is paginated, though, so in practice it only ever pre-passes a page's
worth of ids, not the full library, and doesn't pay that cost.
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