Design a Peer-to-Peer Money Transfer System

Design a peer-to-peer payment platform. A person can own an account, transfer funds to another person, inspect the history of transfers, and see a balance that is always accurate. A prototype might store state only in memory, but the production design must keep money secure through crashes, repeated attempts, and overlapping requests. Your answer should define the API surface, data model, transfer lifecycle, balance accounting, idempotency rules, concurrency plan, failure…