Add service health monitoring: postgres, node, cAdvisor exporters + dashboard

Exporters:
- postgres_exporter: DB connections, transactions, cache hit ratio, query stats
- node_exporter: host CPU, memory, disk, network
- cAdvisor: per-container CPU and memory usage

PostgreSQL:
- Enable pg_stat_statements for query-level performance tracking
- Track index scans vs sequential scans, cache hit ratio

Dashboard (service-health.json):
- DB: connections, size, transactions/s, slow queries, cache hit ratio, index usage
- Host: disk gauge, CPU, memory, network I/O, disk I/O
- BRouter: request latency p50/p95/p99, container CPU + memory
- All containers: CPU and memory comparison

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infrastructure/prometheus/prometheus.yml

@@ -13,6 +13,18 @@ scrape_configs:
     static_configs:
       - targets: ["planner:3001"]
 
+  - job_name: "postgres"
+    static_configs:
+      - targets: ["postgres-exporter:9187"]
+
+  - job_name: "node"
+    static_configs:
+      - targets: ["node-exporter:9100"]
+
+  - job_name: "cadvisor"
+    static_configs:
+      - targets: ["cadvisor:8080"]
+
   - job_name: "caddy"
     static_configs:
       - targets: ["caddy:2019"]