Review follow-ups to the prune rework.
ImageStore.Remove lost its only production caller when the sweep took over
orphan-file reclamation. It carried its own copy of the mtime guard that now
lives solely in Sweep, so the two could drift, and it stood as an invitation to
grow a second reclaim path. Its specs either duplicated Sweep's own coverage or
tested nothing else, so they go with it; the one unrelated test that used it to
delete a file calls os.Remove directly.
Sweep warned once per file it could not remove, which is fine for a stray
permission bit and useless for a store mounted read-only - a large library would
emit one line per aged file on every prune, and prune would still report success.
Count the failures instead and warn once, with the count and the last error, so a
systemic failure is legible without drowning the log.
Also: the orphan log line counts rows, not files, and now says so; and the
blocked-removal spec asserts its two fixtures really do land in different shards,
since the assertion is vacuous if they collide.
362941fb5c