Add observability stack: ServiceMonitors, Tempo, OTel API instrumentation, dashboards

- Add ServiceMonitors for Traefik, ArgoCD, and Longhorn
- Enable cert-manager ServiceMonitor via helm values
- Deploy Grafana Tempo for distributed tracing (single-binary, Longhorn PVC)
- Add Tempo datasource with trace-to-logs and trace-to-metrics correlation
- Instrument API with OpenTelemetry SDK (Prometheus metrics + OTLP traces)
- Replace console.log with pino structured logging + pino-http middleware
- Add Grafana dashboards for Traefik, API overview, and PostgreSQL (CNPG)

infra/kubernetes/platform/longhorn/kustomization.yaml

@@ -3,3 +3,4 @@ kind: Kustomization
 resources:
   - application.yaml
   - namespace.yaml
+  - servicemonitor.yaml

infra/kubernetes/platform/longhorn/servicemonitor.yaml

@@ -0,0 +1,15 @@
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: longhorn
+  namespace: longhorn-system
+  labels:
+    app: longhorn
+spec:
+  selector:
+    matchLabels:
+      app: longhorn-manager
+  endpoints:
+    - port: manager
+      interval: 30s
+      path: /metrics