resizedItem is the only implementation of artworkReader, which is
declared in image_cache.go, and it is used by the worker's precache as
well as the serving path -- so worker.go was reaching into serving.go
for a cache type. representationTag stays in serving.go, where the
HTTP validator belongs.
Pure move.
artworkReader.LastUpdated had no callers: invalidation rides entirely on
the cache key, so the interface member, the resizedItem field and its
four assignments were vestigial. Reader's second return value was
likewise discarded at all three call sites.
resizedItem.Key duplicated representationTag's format string over
identical inputs, where drift would serve a wrong-keyed entry under a
right-looking validator; it now derives from it. newResizedItem had one
caller and a doc comment claiming a sharing with worker.precache that
never existed -- precache builds its own literal.
Also unexport Prune, which no caller outside the package used while
RunPrune documented itself as the only sanctioned path, drop a
single-call placeholder wrapper, and delete five fakeFolderRepo fields
no spec ever set.
