Capstone: Design a Payment API

Payment operations require clear rules for retries, permissions, uncertain outcomes, and concurrent changes.

A timed-out create may charge twice. A valid credential may still belong to the wrong merchant. A processor may receive a request and never answer. Two support agents may refund the same remaining amount at the same time. A cached response may show money that is no longer available.

General terms such as secure or reliable do not explain what callers should do. Define specific responses and recovery steps.