Add scheduled ntfy reminders for overdue restore tests

[borg-codex-bot]

Summary

Add a dedicated reminder mechanism for overdue scheduled restore tests. The current ntfy restore_test_overdue event is only emitted when a scheduled restore-test run starts and detects a due job. It does not act as an independent reminder if a job is overdue but no scheduled run has executed yet.

Current behavior

• The UI can show restore tests as overdue.
• ntfy does not poll independently.
• restore_test_overdue is only sent during /api/restore-tests/run with scheduled=true.
• Manual restore-test runs do not send overdue notifications.
• If no restore-test cron/schedule runs, no ntfy reminder is sent for an overdue restore test.

Desired behavior

Introduce a background/scheduled overdue reminder check for restore tests:

• Periodically evaluate scheduled restore-test policies.
• Detect jobs whose restore test is overdue.
• Send an ntfy notification for overdue scheduled restore tests, even when no restore test is being executed immediately.
• Avoid notification spam by sending at most one reminder per job per overdue period or by using a configurable reminder interval.
• Persist reminder state so service restarts do not cause duplicate notifications.
• Keep manual runs silent unless explicitly intended.

Suggested implementation notes

• Reuse the existing restore-test plan calculation where possible.
• Add a small persisted state file for reminder delivery, for example under the runtime config/state directory.
• Consider a cron-backed endpoint or a lightweight scheduler route similar to the existing restore-test schedule path.
• Only notify for jobs with policy mode scheduled and is_overdue=true.
• Reset or clear the reminder state after a successful restore test for that job.
• Respect ntfy event selection and only send when restore_test_overdue is enabled.

Acceptance criteria

• Overdue scheduled restore tests can trigger ntfy reminders without starting a restore test immediately.
• A user does not receive repeated notifications on every poll for the same overdue period.
• Reminder state survives UI/server restarts.
• Successful restore tests clear the overdue reminder state for the affected job.
• The behavior is documented in the UI/help or release notes.
• Secrets are not logged.

User impact

Users with scheduled restore-test policies can be informed when a restore verification is overdue, instead of only seeing the overdue state in the UI.