A full scan re-imports every track, so the per-entity artwork enqueue in
persistChanges fired for every album and artist in the library — measured as
100% of both on a live instance, and ~8.5k artists / ~16.7k Deezer fetches on a
96k-file library. Re-importing a track is no evidence the art changed: artist
art has no track-content source at all, and the albums re-derived to identical
hashes across three consecutive scans.
Enqueue through EnqueueIfMissing on a full scan, which anti-joins item_artwork
so only entities that never resolved are queued. Incremental scans keep the
unconditional Enqueue, where a re-import does mean the file changed. Doing this
at the enqueue site rather than skipping it wholesale keeps a first scan filling
in covers as it walks, instead of stalling every cover until the scan ends.
EnqueueMissing grows a priority argument and runs once at end of scan, so an
entity phase 1 never saw still resolves, at Scan priority rather than dropping
to Recheck behind the backfill.
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