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).

demo-app/Dockerfile

@@ -0,0 +1,15 @@
+# demo-app api (Kubernetes layer of the self-healing lab). Built with Cloud Build into the public Artifact Registry repo.
+FROM golang:1.27.1-alpine AS build
+WORKDIR /src
+ENV CGO_ENABLED=0 GOFLAGS=-mod=mod
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN go build -trimpath -ldflags="-s -w" -o /out/demo-api .
+
+FROM alpine:3.22
+RUN apk add --no-cache ca-certificates && adduser -D -u 10001 app
+USER 10001
+COPY --from=build /out/demo-api /usr/local/bin/demo-api
+EXPOSE 8080
+ENTRYPOINT ["/usr/local/bin/demo-api"]