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/compose/observability.yml

@@ -35,6 +35,27 @@ services:
       - PROMETHEUS_SERVER_URL=http://otel-collector:8889
       - LOG_LEVEL=info
 
+  # ── Prometheus (Metrics Backend) ──
+  prometheus:
+    container_name: prometheus
+    image: prom/prometheus:latest
+    restart: always
+    networks:
+      app-shared-net:
+        aliases:
+          - prometheus
+    ports:
+      - '9090:9090'
+    command:
+      - '--config.file=/etc/prometheus/prometheus.yml'
+      - '--storage.tsdb.path=/prometheus'
+      - '--web.console.libraries=/etc/prometheus/console_libraries'
+      - '--web.console.templates=/etc/prometheus/consoles'
+      - '--web.enable-lifecycle'
+    volumes:
+      - ../../infra/otel/prometheus.yml:/etc/prometheus/prometheus.yml:ro
+      - prometheus_data:/prometheus
+
   # ── Custom Dashboard (nginx:alpine) ──
   dashboard:
     container_name: dashboard
@@ -57,6 +78,9 @@ services:
       otel-collector:
         condition: service_started
 
+volumes:
+  prometheus_data:
+
 networks:
   app-shared-net:
     name: app-shared-net