feat(infra): add Prometheus and Chart.js dashboard with real-time charts

- Add Prometheus server scraping OTel collector
- Add Chart.js donut chart for service health distribution
- Add line charts for RPS, latency, error rate, trace volume
- Add Prometheus API proxy to nginx whitelist
- Restructure dashboard layout with 12-column responsive grid

infra/dashboard/nginx.conf

@@ -62,10 +62,16 @@ http {
         # OTel collector health
         location /api/health {
             proxy_pass http://otel-collector:13133/;
-            proxy_set_header Host $host;
             proxy_read_timeout 5s;
         }
 
+        # Prometheus API (read-only queries)
+        location /api/prometheus/ {
+            proxy_pass http://prometheus:9090/;
+            proxy_set_header Host $host;
+            proxy_read_timeout 15s;
+        }
+
         # Docker API proxy — strict whitelist (read-only Unix socket)
         # Uses exact match (=) to avoid regex+proxy_pass URI limitation.
         location = /api/docker/containers/json {