fix(中间件): 修复演示环境中间件的权限检查逻辑

修改演示环境中间件的权限检查流程，先执行请求再检查用户权限，避免误拦截合法请求

backend/app/core/middlewares.py

@@ -88,8 +88,21 @@ class DemoEnvMiddleware(BaseHTTPMiddleware):
                 path in settings.DEMO_WHITE_LIST_PATH or 
                 (user_username and user_username in settings.DEMO_USER_WHITE_LIST)):
                 return await call_next(request)
+            
             else:
-                return ErrorResponse(msg="演示环境，禁止操作")
+                try:
+                    response = await call_next(request)
+                    
+                    # 现在检查用户是否有权限执行操作
+                    user_username = request.scope.get("user_username")
+                    if user_username and user_username not in settings.DEMO_USER_WHITE_LIST:
+                        # 如果用户没有权限，返回错误响应
+                        return ErrorResponse(msg="演示环境，禁止操作")
+                    
+                    return response
+                except Exception as e:
+                    # 处理可能的异常
+                    raise e
 
         return await call_next(request)