feat: logging enhancements

• Log the caller on every request (52efcb7). CallerContext gets client_id (the token's azp). Once the caller is resolved, tenant_id, app_provider_id and client_id are added to every log line. A middleware writes one http_request line per request: method, route template, status, duration, x-correlator and caller. Health probes log at debug.
• JSON logs (5e7276f). The console renderer is replaced by JSONRenderer, and tracebacks are kept in an exception field.