Wake on-call before orders stop moving

The overnight order consumers stopped draining work, but the queue never grew large enough to trip the current depth alarm. The oldest order waited for twenty minutes before anyone noticed. On-call needs a sustained age alarm and a recovery notification without false pages when the queue is inactive.

Repair monitoring.yaml to notify orders-on-call when the oldest orders-processing message exceeds 300 seconds for three consecutive one-minute periods, and again on recovery.

• For each one-minute period, use the highest reported oldest-message age so lower observations cannot hide a breach.
• Missing data must not trigger a page.
• Preserve the queue, notification topic, existing traffic relationship, and queue retention and visibility settings.