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/docker-compose.yml

@@ -78,12 +78,52 @@ services:
       POSTGRES_DB: trails
     volumes:
       - pgdata:/var/lib/postgresql/data
+    command:
+      - "postgres"
+      - "-c"
+      - "shared_preload_libraries=pg_stat_statements"
+      - "-c"
+      - "pg_stat_statements.track=all"
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U trails"]
       interval: 5s
       timeout: 5s
       retries: 5
 
+  postgres-exporter:
+    image: prometheuscommunity/postgres-exporter:latest
+    restart: unless-stopped
+    environment:
+      DATA_SOURCE_NAME: postgresql://trails:${POSTGRES_PASSWORD:-trails}@postgres:5432/trails?sslmode=disable
+    depends_on:
+      postgres:
+        condition: service_healthy
+
+  node-exporter:
+    image: prom/node-exporter:latest
+    restart: unless-stopped
+    pid: host
+    volumes:
+      - /proc:/host/proc:ro
+      - /sys:/host/sys:ro
+      - /:/rootfs:ro
+    command:
+      - "--path.procfs=/host/proc"
+      - "--path.sysfs=/host/sys"
+      - "--path.rootfs=/rootfs"
+      - "--collector.filesystem.mount-points-exclude=^/(sys|proc|dev|host|etc)($$|/)"
+
+  cadvisor:
+    image: gcr.io/cadvisor/cadvisor:latest
+    restart: unless-stopped
+    privileged: true
+    volumes:
+      - /:/rootfs:ro
+      - /var/run:/var/run:ro
+      - /sys:/sys:ro
+      - /var/lib/docker/:/var/lib/docker:ro
+      - /dev/disk/:/dev/disk:ro
+
   prometheus:
     image: prom/prometheus:latest
     restart: unless-stopped