fix: auto-fix code issues (cron)

- 修复重复导入/字段
- 修复异常处理
- 修复PEP8格式问题
- 添加类型注解

backend/test_phase8_task2.py

@@ -205,7 +205,7 @@ def test_subscription_manager() -> None:
 
         # 更改计划
         changed = manager.change_plan(
-            subscription_id=subscription.id, new_plan_id=enterprise_plan.id
+            subscription_id=subscription.id, new_plan_id=enterprise_plan.id,
         )
         print(f"✓ 更改计划: {changed.plan_id} (Enterprise)")