system: make default nightly backup cron job configurable in some way

Description

When Remote Backup (sFTP) is configured, OPNsense performs an implicit nightly reachability check via configd, independent of the user-defined backup schedule.
This check typically runs around 01:00–02:00, even when the actual backup is scheduled at a different time (e.g. 10:00).
If the remote target is intentionally offline (e.g. NAS powered down overnight), the check fails and logs an error, although the scheduled backup itself works perfectly.
This behaviour is operationally misleading, because:

• the failure is not a scheduled backup
• the user-configured cron job is not involved
• the remote target may be offline by design
• logs show a backup failure even though backups succeed as scheduled

Observed behaviour

• Remote Backup (sFTP) enabled
• Backup scheduled via cron (e.g. daily at 10:00)
• NAS powered off overnight
• Around 01:00–02:00 an sFTP attempt is logged as a failure
• The cron job at 10:00 runs successfully and uploads the config
  The nightly attempt is not visible in System → Settings → Cron and reappears if removed manually from root crontab, indicating it is managed by configd.

Expected behaviour

One of the following (any would solve the issue):

1. Ability to disable the implicit reachability check
2. Make the reachability check follow the user-defined backup schedule
3. Clearly distinguish log severity:

• scheduled backup failure → error
• background reachability probe failure → notice / info

4. Allow a maintenance / blackout window for the remote target
5. At minimum, log the event clearly as a background health check, not a backup failure

Why this matters

• Produces false-positive error logs
• Reduces trust in backup alerts
• Common setup (NAS power saving is normal)
• No functional issue, but poor signal-to-noise ratio
• Difficult to troubleshoot because it looks like a real backup failure

Environment

• OPNsense version: 25.7.9
• Remote Backup: sFTP
• Target: Synology NAS (powered down overnight)
• Backup itself works correctly when target is online

Additional notes

This is not a request to remove the check entirely — only to make it explicit, configurable, or schedule-aware, so that logs accurately reflect real backup failures.