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/Caddyfile

@@ -1,3 +1,10 @@
+{
+	servers {
+		metrics
+	}
+	admin 0.0.0.0:2019
+}
+
 (security_headers) {
 	header {
 		Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"