Update README with logging & request-ID details

[mattmorgis]

Depends on: #6

Context

We just added request-scoped logging with X-Request-ID propagation. The README should document how it works so future contributors & users understand the behaviour and how to correlate logs.

Task

1. Add a new section ### Logging & Request Correlation in README.md covering:
   • Log format: %(asctime)s [%(levelname)s] [%(request_id)s] ...
   • Example access log line and application log line.
   • Explanation that each response includes X-Request-ID and how clients can use it.
   • Note that we use stdlib only (no structlog) and contextvars makes it async-safe.
2. Update the top-level feature list / bullet points to include “Per-request UUID logging”.

Acceptance criteria

• README.md builds without markdown lint errors.
• New section clearly explains the above bullet points.

Notes

Docs-only change; no code.