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/otel/prometheus.yml

@@ -0,0 +1,12 @@
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+
+scrape_configs:
+  - job_name: 'otel-collector'
+    scrape_interval: 10s
+    metrics_path: /metrics
+    static_configs:
+      - targets: ['otel-collector:8889']
+        labels:
+          service: otel-collector