Consider snapper snapshots for all repos

[jernst]

To be created right after an rsync upload ends.
To be browseable at a particular URL, say depot.ubos.net/history, with archives at depot.ubos.net/history/<snapshot>/<repo>.

[jernst]

This approach isn't practical any more since moving to S3. Perhaps we could just create additional repo.tar.xz files with sequence number in it, such as repo-12, in the same directory, and update the purge algorithm so that old package versions remain around.