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/schemas/exam.py

@@ -244,7 +244,7 @@ class RecentExamItem(BaseModel):
     total_score: float = Field(..., description="总分")
     is_passed: Optional[bool] = Field(None, description="是否通过")
     duration_seconds: Optional[int] = Field(None, description="考试用时(秒)")
-    start_time: str = Field(..., description="开始时间")
+    start_time: Optional[str] = Field(None, description="开始时间")
     end_time: Optional[str] = Field(None, description="结束时间")
     round_scores: RoundScores = Field(..., description="三轮得分")