feat: Add request ID propagation middleware

[Nexussyn]

Summary

Implements request ID propagation for distributed tracing:

• Adds middleware that extracts or generates X-Request-ID header
• Stores request ID in request extensions for downstream access
• Includes response header with request ID for client correlation

Closes #17

[Nexussyn]

✅ Ready for merge — All issues resolved:

• Fixed REQUEST_ID_HEADER constant that contained garbage characters — now correctly set to "X-Request-ID"
• Fixed tower::Service impl signature (removed invalid Into<http::Response<()>> bound)
• Added pub mod middleware export in lib.rs
• Middleware logic is complete and tests pass

This PR is ready for review and merge.