Design a robust idempotency mechanism for a RESTful payment API that processes millions of transactions per day. Your solution must handle network failures, timeouts, and duplicate requests without double-charging users. Explain how you would generate and store idempotency keys, handle TTL (Time To Live) for these keys to prevent memory bloat, and ensure consistency across a distributed database cluster. Discuss the trade-offs between using idempotency keys at the application layer versus the infrastructure layer, and provide pseudo-code for the middleware that would enforce this logic.