Each viewer reports command round trip, camera fps and frame age, and the owner sees them live under Admin -> Health #89

Part of #54 items 4-5. Needs the server counters and the camera socket first.

Done:

• [ ] The page measures command -> pose round trip (p50/p95), commands sent/dropped per second, camera fps received, frame age, stalls and reconnects, and sends a small summary every 5 s over the existing /live socket (validated at the boundary, size-capped).
• [ ] src/live-metrics.ts aggregates them per viewer and exposes them in /metrics.prom.
• [ ] The admin page (src/admin-page.ts) gets a Health section with those numbers per viewer, refreshed live, escaped.
• [ ] Tests: a forged or oversized report is refused; the aggregation reports the injected values.