Average latency masks event-loop stall tail latency

Average latency metrics hide tail latency spikes from event-loop stalls. Stalls create queueing and spikes that users experience as freezes, but averages get pulled down by fast requests.

Alert on p95/p99 latency metrics instead of average. Add event-loop lag p95, time-to-first-byte (TTFB), and server-side request queue length or in-flight request count.