feat: add demo-app api service

실험·앱 계층 설계서 3.1의 demo-app api(컨테이너 계층). /healthz, /readyz(DB ping +
legacy /healthz), /api/orders GET/POST(app.orders), /api/price/{item}(legacy-api via ILB,
LEGACY_TIMEOUT 기본 2s), /metrics(demo_http_*). OTLP 트레이스는 OTEL_EXPORTER_OTLP_ENDPOINT
설정 시 전송(otelhttp). 이미지는 공개 Artifact Registry selfheal-public(노드 익명 pull).

README.md

@@ -7,7 +7,7 @@ Infrastructure: `seon-labs/selfheal-lab-terraform`. Runtime configuration (compo
 | Directory | Runs on | Image |
 |---|---|---|
 | `legacy-api/` | GCP MIG instances (docker compose) | Artifact Registry `asia-northeast1-docker.pkg.dev/cms-d-414903/selfheal/legacy-api:<git sha>` |
-| `demo-app/` | AWS Kubernetes (Argo CD) | ECR `selfheal/demo-app` (added in WP-14) |
+| `demo-app/` | AWS Kubernetes (Argo CD, namespace demo) | Public Artifact Registry `asia-northeast1-docker.pkg.dev/cms-d-414903/selfheal-public/demo-api:<git sha>` (anonymous pull; kubeadm nodes have no cloud credentials) |
 
 Build (Cloud Build, from the repository root):