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.
Correct the inverted workerDeps.extGate comment, trim over-budget doc comments, and add conf.Server.ArtistImageFolder to the resolution fingerprint so an image-folder change re-resolves artist artwork.
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.
