fix: ExamService.start_exam返回ID避免懒加载

修改start_exam返回exam.id而不是整个Exam对象,
彻底避免SQLAlchemy异步会话的懒加载问题

backend/app/services/exam_service.py

@@ -20,7 +20,7 @@ class ExamService:
     @staticmethod
     async def start_exam(
         db: AsyncSession, user_id: int, course_id: int, question_count: int = 10
-    ) -> Exam:
+    ) -> int:
         """
         开始考试
 
@@ -31,7 +31,7 @@ class ExamService:
             question_count: 题目数量
 
         Returns:
-            Exam: 考试实例
+            int: 考试ID
         """
         # 检查课程是否存在
         course = await db.get(Course, course_id)
@@ -93,8 +93,10 @@ class ExamService:
         db.add(exam)
         await db.commit()
         await db.refresh(exam)
-
-        return exam
+        
+        # 返回exam.id而不是整个对象，避免懒加载问题
+        exam_id = exam.id
+        return exam_id
 
     @staticmethod
     async def submit_exam(