Adds FreshHours and StaleHours settings to control when the dashboard
backup indicator shows green (fresh), amber (stale), or red (critical).
- Backend: Added FreshHours/StaleHours to BackupAlertConfig (default 24/72 hours)
- Frontend: getBackupInfo() now accepts optional thresholds parameter
- Dashboard/GuestRow components use thresholds from alert config
- Settings saved/loaded with alert configuration
Closes#839