Add retention/pruning to backup-gdrive-to-hdd (backupYes disk)

gdrive-backup dest (/media/rob/backupYes/gdrive-backup, mirrors /gdrive/backup via backup-gdrive-to-hdd.timer) has no retention/pruning: whey/*.tar.gz daily state snapshots grow unbounded (16 files/78M as of 2026-09-28, oldest Sep 11 kept indefinitely); lezama/ postgres dumps at 2.5G with dirs back to 2025-11-13. Add age/count-based pruning (e.g. keep last N daily whey-state tarballs, cap lezama dump retention) to backup-gdrive-to-hdd.sh or a companion prune step. Trigger: Elazar noticed disk hit 99% full 2026-09-26/27 from unrelated torrent download landing on same disk; once freed, reviewed gdrive-backup and found 3 stale one-off files already trashed (2 old mars dumps, 1 lezama prewipe snapshot, ~300M) but no ongoing prune mechanism exists.