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/argocd/kustomization.yaml

@@ -8,6 +8,7 @@ resources:
   - appsets/platform.yaml
   - appsets/apps.yaml
   - appsets/previews.yaml
+  - servicemonitor.yaml
 patches:
   - target:
       kind: ConfigMap