In the window before a rescan records a deleted cover (or after cache
eviction/restart), no row or mtime signal moves, so the freshness skip kept
the stale hash. ErrUnavailable serves now carry a sourceGone signal that
bypasses the skip only while a stored hash remains to clear — afterwards
the negative cache applies, so artwork-less entities still don't re-resolve
per serve. The e2e spec now covers the no-rescan window.
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