feat: add ingress-nginx, demo-app, Chaos Mesh apps and Grafana alert rules

WP-14(Terraform 설계서 7.3, 실험·앱 설계서 3.1·4.3).
- apps/ingress-nginx: baremetal 매니페스트(controller-v1.15.1)를 kustomize로 가져와
  NodePort 30080/30443 고정(ALB TG와 일치).
- apps/demo-app + workloads/demo-app: front(nginx-unprivileged 1.31.6, 정적 페이지 +
  /api/ 프록시) 2, api(demo-api:8f5378b) 3 + PDB minAvailable 2, Service, Ingress(/ front,
  /api api). DB_DSN은 Secret demo-db(make k8s-secrets로 생성, git 밖).
- apps/chaos-mesh: Helm chart 2.8.4(containerd 소켓, dashboard 없음).
- vm/observability/grafana/provisioning/alerting: contact point harness-webhook
  (10.20.0.11:8080/hooks/alert), 루트 정책, 규칙 8건(ProbeDown·ApiLatencyHigh·
  ApiErrorRateHigh·PodCrashLooping·LegacyUnhealthy·DbReplicationLag·DbPrimaryDown·
  GitOpsDrift, 라벨 layer/signature).

apps/demo-app.yaml

@@ -0,0 +1,21 @@
+# demo-app (front + api) in namespace demo (experiment design 3.1). DB_DSN comes from Secret demo-db (make k8s-secrets).
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: demo-app
+  namespace: argocd
+spec:
+  project: default
+  source:
+    repoURL: https://git.seonology.com/selfheal-lab/selfheal-lab-gitops.git
+    targetRevision: main
+    path: workloads/demo-app
+  destination:
+    server: https://kubernetes.default.svc
+    namespace: demo
+  syncPolicy:
+    automated:
+      prune: true
+      selfHeal: true
+    syncOptions:
+      - ServerSideApply=true