Issue 184 made the backup check its lock before each original. The
workers that now back up several originals at once take the next file
without checking it. With five downloads waiting, backup.lock is removed
and proper-lockfile's timer finds it gone. The backup must download no
other photo, and reject with "lost the lock on DIR" once those five are
in place. Today its workers go on to the other three photos.
Model: opus-5-5