[Feature] Add Idempotency to Emergency Request Creation #10

Problem

Creating an emergency request twice with same parameters creates duplicates (no idempotency key).

Impact

• Accidental duplicate requests possible
• System inconsistency
• Billing/audit issues

Solution

Implement idempotency key tracking:

• Accept Idempotency-Key header
• Check existing request before creation
• Return cached response if duplicate

Acceptance Criteria

• Idempotency-Key header accepted
• Duplicates detected and returned from cache
• Request deduplication tested
• 24-hour key expiration implemented

Related

Shared util: shared/utils/idempotency.js