fix(deploy): 为 docker-compose 添加 image 标签以对齐 CI/CD 流程

switch.sh 依赖 reading-platform-backend:latest 和
reading-platform-frontend:latest 镜像名来启动容器，
不加 image 字段时 docker compose build 会生成错误的镜像名
导致 CI/CD 部署后实际运行的仍是旧镜像

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

docker-compose.yml

@@ -2,6 +2,7 @@ version: '3.8'
 
 services:
   backend:
+    image: reading-platform-backend:latest
     build:
       context: ./reading-platform-java
     container_name: kg-backend
@@ -19,6 +20,7 @@ services:
       - uploads_data:/app/uploads
 
   frontend:
+    image: reading-platform-frontend:latest
     build:
       context: ./reading-platform-frontend
     container_name: kg-frontend