Design a payment system with retries and multiple payment methods

Problem

Design a payment system supporting multiple payment methods, durable execution, and retries, covering both high-level and low-level design.

Requirements

• Multiple payment methods
• Explicit payment states and transitions
• Durable execution with safe retries

Discussion points

• HLD components mapped directly onto LLD entities
• Strategy pattern for state and payment-method handling
• Ensuring retries do not double-charge (idempotency keys)