Write a runbook for someone woken at 3am

An alert exists and the only person who knows what to do is on holiday.

Write an on-call runbook for the alert below, for a responder who did not build this system and is half awake.

Structure:
1. **What this alert means**, one sentence, plus what users are experiencing right now.
2. **Severity call**: how to decide in under a minute whether this needs escalation or can wait until morning.
3. **First checks**, in order, each with the exact command or dashboard link and what a healthy result looks like.
4. **Most likely causes**, ranked by frequency, each with its confirming check and its fix.
5. **Mitigations** that buy time without fixing the cause: restart, scale, disable a feature flag, fail over. State the side effects of each.
6. **Escalation**: who to wake, and the threshold for waking them.
7. **After**: what to capture for the follow-up while it is still happening.

No step may require judgement the responder does not have. Where a step is destructive, mark it clearly.

Alert and system:

Runbooks and support. What to do at 3am, the failures that keep arriving, and the limits nobody wrote down.