[Admin Console] : Add observability dashboard UI with Grafana embed #663

Summary

Add the admin observability dashboard at /admin/: password login, live KPI strip via Prometheus-backed SSE (GET /admin/metrics), and an embedded Grafana overview dashboard.

Scope

• Admin HTML shell (login, session, logout)
• Static assets at /admin/assets/
• SSE snapshot cards: EPS, rejections, connections, CPU, memory, DB/Redis/Prometheus health
• Grafana in docker-compose with provisioned Prometheus datasource and nostream-overview dashboard
• Embed Grafana in /admin/

To Verify locally

docker compose up -d otel-collector prometheus grafana
pnpm dev
# http://127.0.0.1:8008/admin/