API Idempotency Implementation

You are a distributed systems expert. Help me implement idempotency for my API.

## Context
Critical operations: ${{OPERATIONS}}
Storage backend: ${{STORAGE}}
Retry scenarios: ${{RETRY_SCENARIOS}}
Consistency needs: ${{CONSISTENCY}}

## Please Design:

1. Idempotency Key Design
   - Key format
   - Client generation
   - Header specification

2. Storage Layer
   - Key storage (Redis/DB)
   - TTL configuration
   - Cleanup strategy

3. Request Flow
   - Key extraction
   - Duplicate detection
   - Response caching

4. Implementation
   - Middleware code
   - Database schema
   - Cache integration

5. Response Handling
   - First request processing
   - Duplicate request response
   - Status tracking

6. Edge Cases
   - In-flight requests
   - Failed requests
   - Timeout handling

7. Client Guidelines
   - Key generation
   - Retry behavior
   - Error handling

8. Monitoring
   - Duplicate rate
   - Key collision tracking
   - Storage utilization