fix: 修复考试API路由冲突和响应验证问题

1. 调整路由顺序：将/records和/statistics放在/{exam_id}之前
2. 修复RecentExamItem.start_time允许None值

backend/app/api/v1/exam.py

@@ -260,20 +260,6 @@ async def get_mistakes(
         )
 
 
-@router.get("/{exam_id}", response_model=ResponseModel[ExamDetailResponse])
-async def get_exam_detail(
-    exam_id: int,
-    db: AsyncSession = Depends(get_db),
-    current_user: User = Depends(get_current_user),
-):
-    """获取考试详情"""
-    exam_data = await ExamService.get_exam_detail(
-        db=db, user_id=current_user.id, exam_id=exam_id
-    )
-
-    return ResponseModel(code=200, data=ExamDetailResponse(**exam_data), message="获取成功")
-
-
 @router.get("/records", response_model=ResponseModel[dict])
 async def get_exam_records(
     page: int = Query(1, ge=1),
@@ -304,6 +290,21 @@ async def get_exam_statistics(
     return ResponseModel(code=200, data=stats, message="获取成功")
 
 
+# 注意：动态路由 /{exam_id} 必须放在固定路由之后
+@router.get("/{exam_id}", response_model=ResponseModel[ExamDetailResponse])
+async def get_exam_detail(
+    exam_id: int,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """获取考试详情"""
+    exam_data = await ExamService.get_exam_detail(
+        db=db, user_id=current_user.id, exam_id=exam_id
+    )
+
+    return ResponseModel(code=200, data=ExamDetailResponse(**exam_data), message="获取成功")
+
+
 # ==================== 试题生成接口 ====================
 
 @router.post("/generate", response_model=ResponseModel[GenerateExamResponse])