Phase 3.5: Implement idempotency support

[copyleftdev]

Description

Add idempotency key support for safe request retries.

Tasks

• Accept Idempotency-Key header
• Store idempotency keys in database
• Check for duplicate requests
• Return cached response for duplicate keys
• Implement key expiration (24 hours)

Documentation

• See docs/02-api-specification.md for idempotency spec
• See docs/03-system-architecture.md Section 9 for failure handling

Acceptance Criteria

• Duplicate requests with same key return same result
• Keys expire after 24 hours
• Works with both sync and async requests