admin: backup logs via parameters.php, nextcloud secondary backup

- surface backup/cleanup cron logs + backup freshness status
- email xamxam@erg.be when SQLite backups go stale (backup watchdog)
- sync SQLite snapshots to Nextcloud WebDAV + remote-freshness watchdog
- precise retention pruning, manual sync in check recipe, and Nextcloud-sync docs

docs/deployment.md

@@ -95,6 +95,9 @@ gzipped to `/var/backups/xamxam/` on the server.
 Retention: hourly backups kept 30 days, nightly (02:00) backups kept 90 days.
 Backup files: `/var/backups/xamxam/db-<timestamp>.db.gz`.
 
+Off-site copies are synced daily to Nextcloud WebDAV — see
+[nextcloud-sync.md](nextcloud-sync.md).
+
 Draft cleanup is handled by a separate cron (`/etc/cron.d/xamxam-cleanup`),
 installed via `just deploy-cleanup-cron`, logging to
 `/var/log/xamxam-cleanup-YYYY-MM-DD.log`. Verify with `just deploy-check-cleanup-log`.