With CoverArtPriority="external,cover.jpg", a transient external failure
followed by a folder hit dropped the external error: the worker recorded
found and deleted the queue row, so the configured higher-priority external
art was never retried. Carry extError onto the fallback resolution and add
an outcomeFoundStale that persists+serves the art but reschedules via
MarkFailed, giving the external source another chance. When external later
answers definitively-not-found, the hit is not stale and the row is deleted.
A user-editable ExternalImageURL can point at an arbitrarily large endpoint;
a fast server could make the worker buffer hundreds of MB inside the 5s HTTP
timeout. Bound the read to a fixed 20MB cap (no config knob) via io.LimitReader
and fail the item if it is exceeded.
Resolves one queue item end to end: hash/dedup, decode + 128px thumbnail
blurhash, place bytes (store vs source file), and persist found/absent/
failed state for the worker (Task 4) to act on.
