POST /images/thumbnails/plan reviews a move (what is copied, removed and
left, free space, too-long Windows paths, sources no longer listed) and
hands out a token; POST /jobs/thumbnails {mode: move} runs it as a
thumbnails job: hold automatic previews, copy and check every preview on
the job pool, switch with one config commit that records the old copies,
then remove them. Per-file errors still switch; a folder-wide error, a
cancel before the switch or a crash leaves nothing changed (a journal is
undone at the next start); old copies that could not go are removed by a
finishing job at the next start. Leaving a folder removes the previews of
this install's sources that are no longer listed (Q320-8).
Recreate folder and Use this folder answer an unavailable folder; the
Thumbnail maintenance scope prune removes the thumbnails of removed
sources.
