fix(admin-service): Dockerfile 预建 /app/uploads 并 chown node:node

Docker named volume 首次挂载时会继承镜像中目标目录的属主。
若目录不存在或属主为 root，容器以 node(1000) 运行时无写权限。
在 USER node 之前创建目录并设置正确属主即可解决。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/admin-service/Dockerfile

@@ -27,6 +27,7 @@ RUN apk add --no-cache dumb-init
 COPY --from=builder /app/services/admin-service/dist ./dist
 COPY --from=builder /app/services/admin-service/node_modules ./node_modules
 COPY --from=builder /app/services/admin-service/package.json ./
+RUN mkdir -p /app/uploads && chown node:node /app/uploads
 USER node
 EXPOSE 3012
 HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \