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)

1 files changed, 12 insertions, 4 deletions

diff --git a/internal/metrics/metrics.go b/internal/metrics/metrics.go
index 9030775..74f9ffb 100644
--- a/internal/metrics/metrics.go
+++ b/internal/metrics/metrics.go
@@ -288,12 +288,20 @@ const (
 //go:embed dashboard.html
 var dashboardHTML []byte
 
-func (m *Metrics) Serve(addr, path string) error {
-        mux := http.NewServeMux()
-        mux.Handle(path, promhttp.Handler())
-        mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+func (m *Metrics) PrometheusHandler() http.Handler {
+        return promhttp.Handler()
+}
+
+func (m *Metrics) DashboardHandler() http.Handler {
+        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                 w.Header().Set("Content-Type", "text/html; charset=utf-8")
                 w.Write(dashboardHTML)
         })
+}
+
+func (m *Metrics) Serve(addr, path string) error {
+        mux := http.NewServeMux()
+        mux.Handle(path, m.PrometheusHandler())
+        mux.Handle("/", m.DashboardHandler())
         return http.ListenAndServe(addr, mux)
 }