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Opaque client-generated identifier (recommend 36-character UUID) that lets the server detect retried requests on this mutation endpoint and return the cached prior response instead of re-running side effects. Implements the IETF idempotency-header draft (https://datatracker.ietf.org/doc/draft-ietf-httpapi-idempotency-key-header/). Constraints: 16-128 printable ASCII characters. The server returns the same status, body, and Idempotency-Replayed: true on a successful match. If the same key is reused with a different request body, the server returns 422 with error=idempotency-mismatch. Keys expire after 24 hours. Idempotency is opt-in: requests without this header behave exactly as before.