feat: add structured per-request logging with client IP
Add tower-http TraceLayer to log every request with method, path,
client_ip (from x-forwarded-for/x-real-ip/connect_info), status,
and latency_ms. Enables tracing requests across LB, gatekeeper,
and PDS.
Set GATEKEEPER_LOG_FORMAT=json for JSON structured logs (default
is human-readable). Pair with RUST_LOG to control verbosity.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>