Clarify TTL requirements and provide implementation guidance

[alivadjid]

Need clear TTL specifications and expiration behavior

Problem:

Current draft lacks concrete TTL requirements, leading to:

• Implementation inconsistency across providers
• Uncertainty about key expiration behavior
• Storage planning difficulties

Proposed solutions:

1. Define minimum/maximum TTL ranges
2. Specify expiration behavior (410 Gone vs new request)
3. Add optional TTL header: Idempotency-TTL: 3600
4. Clarify "reasonable time" with concrete examples

Use cases:

• Financial operations: 72h TTL
• Order processing: 24h TTL
• Real-time operations: 1h TTL