Access log trace ID correlation

[shaneutt]

Inject trace ID into structured JSON access log entries for log-to-trace correlation.

Requirements

• Include trace_id field in JSON access log entries
• Include span_id field for the root request span
• Trace ID format: 32-character hex string (W3C format)
• Field omitted when tracing is not enabled
• Correlate with OTLP exported traces

Implementation Notes

• Trace ID sourced from current span context
• Enables log-to-trace correlation in observability backends

Parent epic: #10