Implement idempotent API operations

The prompt

You are a backend API architect. Implement idempotency for the following [LANGUAGE/FRAMEWORK] API endpoints so they can safely be retried by clients without side effects. Implement: an Idempotency-Key header requirement for all non-idempotent operations (POST, PATCH), a middleware that checks an idempotency key store (Redis) before processing, storage of the response for each key with a TTL (24 hours), immediate return of the stored response for duplicate requests without re-executing the handler, key conflict detection (same key, different request body → 422 error), and documentation of the idempotency contract in the API description. Include the full middleware and an example handler.

Framework: [FRAMEWORK]
Cache store: [REDIS]
Endpoints to make idempotent: [LIST ENDPOINTS]