Entity-level artwork queries now take a typed model.Kind instead of a bare
prefix string. GetItemArtwork, DeleteForItem(s), GetInfoForItems,
EnqueueStaleAbsent, hydrateItemImages, enqueueBackfillKind and artwork.Refresh
convert to the prefix string only at the two real boundaries: the SQL
item_kind column (kind.Prefix() inside each repo) and external string inputs
(a new model.ParseKind for the nativeapi URL param, which also validates it).
The Backfill/stale-absent kind slices, the resolve.go dispatch switch, and the
kind→resource / kind→table lookup maps now use the Kind vars directly. The
queue lifecycle methods (MarkFailed/Delete*) keep string kinds — they operate
on a dequeued item's raw ItemKind column, which stays a string field, always
populated via kind.Prefix().
Removes every bare "al"/"ar"/… prefix literal from non-test code (27 -> 0);
behavior is unchanged.
Write falls through to a real write when the liveness touch fails, and sweep retention now matches the recorded mime's extension so obsolete variants are reclaimed.
Sweep now honors an mtime grace window (in-flight acquisitions and temp files), reacquired orphans reset the prune grace window, and the queue mock implements real stale-absent semantics.
