feat: add metrics dashboard HTML page

Add a real-time metrics dashboard accessible at the metrics server root.
The dashboard displays relay statistics in a human-readable format with
auto-refresh every 5 seconds.

Features:
- Active connections and subscriptions
- Request counts (total, success, errors)
- Authentication stats (success/failure)
- Rate limiting hits
- Storage metrics (events, DB size, deletions)
- Clean, dark-themed UI with gradient accents
- Auto-refresh and live uptime counter

The dashboard is embedded using go:embed and served at / while
Prometheus metrics continue to be available at /metrics.

Example: http://localhost:9090/ for dashboard
         http://localhost:9090/metrics for raw metrics

0 files changed, 0 insertions, 0 deletions