The backend now checks the stems folder on disk for every "done" job and
reports "unavailable" when it's missing, replacing the old client-side
heuristic that only reacted to a 404 on the single-job endpoint and missed
the case where the registry entry survived but the folder did not. Desktop
shows a yellow "click to reimport" warning wired to the existing
importFromUrl restore path; mobile gets the same detection and one-tap
reimport from scratch, since it had none before.
Closes#380
Thales
0226907255