Implemented axum middleware that generates a UUIDv7 per inbound request
with an 8-character hex prefix exposed as X-Request-Id response header.
- Added RequestId newtype wrapper for type-safe extension access
- request_id_middleware generates UUIDv7, hashes to 8-char hex ID
- Stores in Request extensions for handler access
- Preserves existing x-request-id header if present
- Wire into main router via middleware layer
Acceptance:
- Every response includes X-Request-Id: <8-char hex>
- Request.extensions().get::<RequestId>() works from handlers
- Unit tests verify uniqueness across consecutive requests
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
