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/observability/kube-prometheus-stack/grafana-datasources.yaml

@@ -15,3 +15,22 @@ data:
         url: http://loki.observability.svc:3100
         jsonData:
           maxLines: 1000
+          derivedFields:
+            - datasourceUid: tempo
+              matcherRegex: '"traceID":"(\w+)"'
+              name: TraceID
+              url: "$${__value.raw}"
+      - name: Tempo
+        type: tempo
+        uid: tempo
+        access: proxy
+        url: http://tempo.observability.svc:3100
+        jsonData:
+          tracesToLogs:
+            datasourceUid: loki
+            filterByTraceID: true
+            filterBySpanID: false
+          tracesToMetrics:
+            datasourceUid: prometheus
+          serviceMap:
+            datasourceUid: prometheus