[Phase 3] Set up structured JSON logging with request tracing

## Description
Structured JSON logging with correlation IDs per request. Request ID in every log line and response header.

## Acceptance Criteria
- [ ] JSON log format in production
- [ ] request_id generated per request
- [ ] request_id included in all log lines for that request
- [ ] X-Request-ID response header

## Feature
Feature: F003 / Phase: 3