production note on push notifications during a traffic spike

We ran into this around push notifications during a traffic spike.

Retries arrived out of order and our fixtures only modeled the happy path.

Sleeping in tests hid the races and slowed CI.

Replaying a small set of recorded payloads found the idempotency bug.

We asserted on stored keys, not on elapsed time.

The suite is shorter and finally recreates the incident we cared about.