Add observability: health endpoints, structured logging, metrics, Grafana stack

Health endpoints:
- /api/health (Journal) and /health (Planner) with DB connectivity check
- Docker healthchecks updated to use app health endpoints

Structured logging:
- Pino with JSON output in production, pretty-print in dev
- Request logging middleware in Planner (method, path, status, duration)
- Replaced console.log/error with structured logger in email and auth flows

Prometheus metrics:
- prom-client with default Node.js metrics + custom histograms/gauges
- /metrics endpoints on both apps
- http_request_duration, planner_active_sessions, brouter_request_duration

Monitoring stack:
- Prometheus, Loki, Grafana containers in docker-compose
- Grafana provisioned with datasources, dashboards, and alert rules
- Caddy access logging (JSON to stdout for Loki)
- grafana.trails.cool with basic auth via Caddy

Dashboards and alerting:
- Overview: request rate, error rate, latency p50/p95/p99
- Planner: active sessions, connected clients, BRouter latency
- Infrastructure: memory, CPU, event loop lag
- Alerts: disk >80%, app down 2min, error rate >5%

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

infrastructure/Caddyfile

@@ -17,6 +17,10 @@
 {$DOMAIN:trails.cool} {
 	import security_headers
 	import block_scanners
+	log {
+		output stdout
+		format json
+	}
 	reverse_proxy journal:3000
 }
 
@@ -24,8 +28,19 @@ www.{$DOMAIN:trails.cool} {
 	redir https://{$DOMAIN:trails.cool}{uri} permanent
 }
 
+grafana.{$DOMAIN:trails.cool} {
+	basicauth {
+		{$GRAFANA_USER:admin} {$GRAFANA_PASSWORD_HASH}
+	}
+	reverse_proxy grafana:3000
+}
+
 planner.{$DOMAIN:trails.cool} {
 	import security_headers
 	import block_scanners
+	log {
+		output stdout
+		format json
+	}
 	reverse_proxy planner:3001
 }