Stop mutating root's crontab. All snapshot and scrub schedules now live
in a single TUI-owned file at /etc/cron.d/pve-tui, with a one-shot
migrate_legacy_crontab() that extracts pre-existing pve-tui blocks from
the crontab on startup.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>