Cleanup Failed Backups

This role installs and runs the cleanback CLI to validate and delete failed Docker backups under /Backups/*/backup-docker-to-local.
Validation is performed via dirval; failures can be removed automatically in a non-interactive service execution.

Behavior

Installs cleanback via pkgmgr.
Runs cleanback (main.py) as a systemd oneshot service.
Executes --all with --yes so failing directories are deleted automatically.
No defaults for runtime knobs:
- CLEANBACK_TIMEOUT_SECONDS (required)
- SYS_SCHEDULE_CLEANUP_FAILED_BACKUPS (required)
Workers (CLEANUP_FAILED_BACKUPS_WORKERS) are derived from Ansible facts only (no arbitrary defaults). Facts must be gathered.