Idempotency and retries

Networks fail. Send an Idempotency-Key header on every POST that creates something (payments, refunds), so retrying can never charge twice.

• The same key with the same body returns the first result, with 200 and Idempotent-Replayed: true.
• The same key with a different body is 409 idempotency_conflict.
• Use something unique per order and action, e.g. order-1042 or refund-1042-1.

reference protects you too: creating a payment with a reference that already has an open or paid payment returns that payment.

Retry 5xx and network errors with back-off. Don't retry 4xx without changing the request.