Review finding P2 on #591: two installs sharing one thumbnail folder could
each read the marker, add a key and replace it, so the last writer
dropped the other's key. write_marker now re-reads, changes and replaces
the marker under a lock file made with O_CREAT | O_EXCL beside it (atomic
on shares, unlike flock); a lock older than 30 s, left by a writer that
died, is taken over, and a wait past 1.5 s fails as a TimeoutError like
an unreachable folder. Tested with three processes adding keys at once.