Design a scalable payment system

Design a payment system that supports moving money between users and paying merchants. Your design should cover: - Core product requirements: user-to-user transfers, merchant payments, refunds/chargebacks (if applicable), payment status tracking, transaction history. - Non-functional requirements: correctness (no double-spend), idempotency, high availability, low latency, auditability, compliance considerations, and scalability. - Money movement model: ledger design (e.g., double-entry), authorization vs capture (if cards), holds, settlement, and reconciliation. - APIs and data model: key endpoints, request/response fields, idempotency keys, and how you represent accounts/balances/transactions. - Failure handling: retries, exactly-once vs at-least-once processing, deduplication, consistency guarantees. - Security and risk: authentication, authorization, encryption, fraud detection hooks, limits, and monitoring. Assume the system may integrate with external payment rails/providers (e.g., card networks, bank transfers). Explain key trade-offs and provide a high-level architecture diagram in words.