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).

vm/observability/grafana/provisioning/alerting/contact-points.yaml

@@ -0,0 +1,11 @@
+# Contact point: the harness runner webhook (experiment design 4.3). Payload is Grafana's standard webhook format.
+apiVersion: 1
+contactPoints:
+  - orgId: 1
+    name: harness-webhook
+    receivers:
+      - uid: harness-webhook
+        type: webhook
+        settings:
+          url: http://10.20.0.11:8080/hooks/alert
+          httpMethod: POST