Fix scheduled backup recovery

[sametbasbug]

Summary

• retain expired idempotency rows that immutable media transform claims still reference
• isolate scheduled maintenance tasks so one failure cannot terminate backup work
• add a 04:00 UTC reconciliation trigger for backup runs stale beyond 30 minutes

Verification

• 168 D1 tests passed
• Astro diagnostics: 0 errors, 0 warnings, 0 hints
• production config: 54 assertions passed
• Actions scope tests passed
• production Worker build and dry-run passed