One idea was to compute this at cleanup time, but this has two problems:
* the UI won't reflect this value. can be good or bad
* the cleaner has no easy way to find out the "parent". I guess we should
change our data structure, if we want to go down this route...
the main motivation is that id can be used in REST API routes. previously,
the id was a path and this had a "/" in it. This made /api/v1/backups/:backupId
not work.
