refactor: 后端代码按环境隔离部署

- test 分支 -> backend-test/app
- staging 分支 -> backend-staging/app
- main 分支 -> backend-prod/app

各环境后端代码独立，互不影响

.drone.yml

@@ -56,7 +56,7 @@ steps:
       command_timeout: 5m
       script:
         - echo "=== [测试环境] 部署后端 ==="
-        - cp -r /root/aiedu/backend/app/* /root/aiedu/kaopeilian-backend/app/
+        - cp -r /root/aiedu/backend/app/* /root/aiedu/backend-test/app/
         - docker restart kpl-backend-dev
         - sleep 5
         - docker ps | grep kpl-
@@ -121,7 +121,7 @@ steps:
       command_timeout: 5m
       script:
         - echo "=== [预生产] 部署后端 ==="
-        - cp -r /root/aiedu/backend/app/* /root/aiedu/kaopeilian-backend/app/
+        - cp -r /root/aiedu/backend/app/* /root/aiedu/backend-staging/app/
         - docker restart kaopeilian-backend
         - sleep 5
         - docker ps | grep kaopeilian-
@@ -193,9 +193,9 @@ steps:
       script:
         - echo "=== [生产] 部署后端 ==="
         - |
-          # 同步后端代码到共享目录
-          cp -r /root/aiedu/backend/app/* /root/aiedu/kaopeilian-backend/app/
-          echo "后端代码已同步到共享目录"
+          # 同步后端代码到生产环境目录
+          cp -r /root/aiedu/backend/app/* /root/aiedu/backend-prod/app/
+          echo "后端代码已同步到生产目录"
           
           # 获取 commit message
           COMMIT_MSG="${DRONE_COMMIT_MESSAGE}"