fix: 修复flake8 lint检查错误

- 删除废弃的 admin_positions_backup.py 备份文件
- 修复 courses.py 缺失的 select 导入
- 修复 coze_gateway.py 异常变量作用域问题
- 修复 scheduler_service.py 无用的 global 声明
- 添加 TYPE_CHECKING 导入解决模型前向引用警告

backend/app/api/v1/coze_gateway.py

@@ -203,25 +203,29 @@ async def send_message(request: SendMessageRequest, user=Depends(get_current_use
                 },
             }
 
-    except CozeException as e:
-        logger.error(f"发送消息失败: {e}")
+    except CozeException as coze_err:
+        logger.error(f"发送消息失败: {coze_err}")
         if request.stream:
-            # 流式响应的错误处理
+            # 流式响应的错误处理 - 捕获异常信息避免闭包问题
+            err_code = coze_err.code
+            err_message = coze_err.message
+            err_details = coze_err.details
+
             async def error_generator():
                 yield {
                     "event": "error",
                     "data": {
-                        "code": e.code,
-                        "message": e.message,
-                        "details": e.details,
+                        "code": err_code,
+                        "message": err_message,
+                        "details": err_details,
                     },
                 }
 
             return EventSourceResponse(error_generator())
         else:
             raise HTTPException(
-                status_code=e.status_code or 500,
-                detail={"code": e.code, "message": e.message, "details": e.details},
+                status_code=coze_err.status_code or 500,
+                detail={"code": err_code, "message": err_message, "details": err_details},
             )
     except Exception as e:
         logger.error(f"未知错误: {e}", exc_info=True)