fix: 修复考试API的SQLAlchemy懒加载问题

在访问current_user属性前先提取到局部变量，避免MissingGreenlet错误

backend/app/api/v1/exam.py

@@ -62,9 +62,13 @@ async def start_exam(
     current_user: User = Depends(get_current_user),
 ):
     """开始考试"""
+    # 先提取用户信息，避免后续懒加载问题
+    user_id = current_user.id
+    username = current_user.username
+    
     exam = await ExamService.start_exam(
         db=db,
-        user_id=current_user.id,
+        user_id=user_id,
         course_id=request.course_id,
         question_count=request.count,
     )
@@ -82,9 +86,9 @@ async def start_exam(
         SystemLogCreate(
             level="INFO",
             type="api",
-            message=f"用户 {current_user.username} 开始考试（课程ID: {request.course_id}）",
-            user_id=current_user.id,
-            user=current_user.username,
+            message=f"用户 {username} 开始考试（课程ID: {request.course_id}）",
+            user_id=user_id,
+            user=username,
             ip=http_request.client.host if http_request.client else None,
             path="/api/v1/exams/start",
             method="POST",
@@ -103,8 +107,12 @@ async def submit_exam(
     current_user: User = Depends(get_current_user),
 ):
     """提交考试答案"""
+    # 先提取用户信息，避免后续懒加载问题
+    user_id = current_user.id
+    username = current_user.username
+    
     result = await ExamService.submit_exam(
-        db=db, user_id=current_user.id, exam_id=request.exam_id, answers=request.answers
+        db=db, user_id=user_id, exam_id=request.exam_id, answers=request.answers
     )
     
     # 获取考试记录以获取course_id
@@ -126,9 +134,9 @@ async def submit_exam(
         SystemLogCreate(
             level="INFO",
             type="api",
-            message=f"用户 {current_user.username} 提交考试（考试ID: {request.exam_id}，得分: {result.get('score', 0)}）",
-            user_id=current_user.id,
-            user=current_user.username,
+            message=f"用户 {username} 提交考试（考试ID: {request.exam_id}，得分: {result.get('score', 0)}）",
+            user_id=user_id,
+            user=username,
             ip=http_request.client.host if http_request.client else None,
             path="/api/v1/exams/submit",
             method="POST",