Event-loop stalls invisible without direct lag measurement

Event-loop stalls (frequent 50-200ms or max >1s) go undetected when event-loop lag is not measured directly. Stalls manifest as tail latency spikes and user-perceived freezes while dashboards show normal status.

Implement event-loop lag measurement using Node's perf_hooks.monitorEventLoopDelay with 20ms resolution. Ship p50, p95, p99, and max delay metrics every 5 seconds. Alert on: max delay >1s (page immediately), p99 >200ms bursts, p95 >50ms sustained.