Write a Blameless Postmortem from an Incident Timeline

Here is a raw timeline of an incident (timestamps and what happened/was observed):

{{timeline}}

Impact: {{impact}}

Write a postmortem with these sections:
1. Summary — two sentences: what broke and the user-facing impact.
2. Timeline — cleaned up, in order, each entry stated as an observed fact ("error
   rate crossed 5%") not an interpretation ("the team noticed things were bad").
3. Root cause — the actual underlying cause, distinct from the trigger. If the
   timeline doesn't contain enough information to state a root cause confidently,
   say what's still unknown instead of guessing.
4. Contributing factors — things that made it worse or slower to detect/resolve,
   stated as system/process gaps, never as a person's mistake.
5. Action items — each one specific and verifiable (a monitoring alert to add, a
   runbook step to change), with no vague items like "improve communication."

Keep the tone blameless throughout: describe what the system/process allowed to
happen, never what a person did wrong.