Retrying writes

Send an Idempotency-Key header (any unique value; a UUID is ideal) on a POST and the write becomes safe to retry. Repeat the request with the same key and you get the original response back, marked Idempotent-Replay: true, instead of a second endpoint or a second key. Keys last 24 hours and are scoped to your API key; reusing one with a different body returns 409.