refactor: serve metrics on main HTTP port instead of separate port

Move metrics dashboard and Prometheus endpoint to the main HTTP server
for simplified deployment and single ingress configuration.

Changes:
- Add PrometheusHandler() and DashboardHandler() methods to Metrics
- Serve /dashboard on main HTTP port (was root on separate port)
- Serve /metrics on main HTTP port (was /metrics on separate port)
- Remove separate metrics server goroutine
- Update logging to show metrics paths on main HTTP port

Benefits:
- Single port/ingress needed for all HTTP traffic
- Simpler reverse proxy configuration
- Dashboard accessible alongside main relay endpoints

Endpoints on port 8080:
- / - WebSocket/index
- /nostr.v1.NostrRelay/* - Connect (gRPC-Web)
- /dashboard - Metrics dashboard (HTML)
- /metrics - Prometheus metrics (text)

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