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

1 files changed, 8 insertions, 0 deletions

diff --git a/internal/metrics/metrics.go b/internal/metrics/metrics.go
index 9030d67..9030775 100644
--- a/internal/metrics/metrics.go
+++ b/internal/metrics/metrics.go
@@ -1,6 +1,7 @@
 package metrics
 
 import (
+        _ "embed"
         "net/http"
 
         "github.com/prometheus/client_golang/prometheus"
@@ -284,8 +285,15 @@ const (
         StatusInvalidRequest  RequestStatus = "invalid_request"
 )
 
+//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) {
+                w.Header().Set("Content-Type", "text/html; charset=utf-8")
+                w.Write(dashboardHTML)
+        })
         return http.ListenAndServe(addr, mux)
 }