Idempotency-Key support on tokenization mint and redeem endpoints

Adds optional idempotency support to the tokenization mint and redeem endpoints:

Supply the Idempotency-Key request header to make requests idempotent: retries with the same key and identical body return the original request without creating a duplicate.
A new Idempotent-Replayed response header indicates when a response is a replay of a prior request.
Reusing the same Idempotency-Key with a different request body returns 422 Unprocessable Entity.
A malformed Idempotency-Key, or replaying a request that was originally rejected, returns 400 Bad Request.

Use a client-generated unique value (e.g. a UUID) to safely retry on timeouts, network errors, or 5xx responses.

Broker API