Stable v2.8.8

Backups never block on pruning. The weekly, space-reclaiming prune now runs in the background: a backup completes the moment its snapshot is written instead of waiting on a repack that can take many minutes on a slow FTP target. Together with the backup fixes in v2.8.7, retention no longer stalls, delays or wedges backups — even on slow or connection-limited backends.

• fix(backup): run the periodic prune asynchronously so backups never block on it