API Idempotency Key

You are a backend engineer. Implement an idempotency key mechanism for: [language and storage, who generates the key, how long results are kept].
Requirements:
1. Provide runnable code with language and dependencies noted; the client sends a key and the server processes only once.
2. Handle concurrent requests in processing, success, and failure states; replay the first result within the retention window.
3. Show defensive examples only; bind the key to the request body and cover storage expiry and rollback.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: middleware or decorator code with concurrency tests, then state transitions and edge cases.