Design a payment system with holds and batching

## Scenario Design a payment system that supports: 1. **Hold payment (authorization)**: Place a temporary hold on a customer’s payment method for a given amount. 2. **Batch process funds (settlement)**: Capture/settle authorized holds in batch (e.g., end-of-day) and move funds to the merchant. 3. **Charge payment (direct charge)**: Charge immediately (auth + capture) when batching is not desired. The system must be reliable, idempotent, and auditable. ## What to cover - APIs and core data model - End-to-end flows (hold, capture/batch settlement, direct charge, cancel/release hold, refunds) - Consistency guarantees and failure handling - Idempotency strategy - Handling duplicates, retries, partial failures - Observability and reconciliation - Security/compliance considerations (high level)