fix(rating): 区间重叠判定改用锁定读取，防并发重叠双提交

先查后写无锁时，两管理员并发提交重叠区间互判不重叠而双双入库
（区间重叠无唯一键可拦，同一总分命中多等级且结果取决于插入顺序）；
判重叠的 list_all 加 for_update 锁全表行串行化并发判定，只读直出
list_grades 不加锁。SQL 形态用例锁定两条口径。

Co-Authored-By: Claude Code <noreply@anthropic.com>

backend/app/services/rating_service.py

@@ -156,7 +156,7 @@ def create_grade(
         new_min = _to_decimal(payload.min_score)
         new_max = _to_decimal(payload.max_score)
         _ensure_score_range(new_min, new_max)
-        _ensure_no_overlap(rating_repository.list_all(session), new_min, new_max)
+        _ensure_no_overlap(rating_repository.list_all(session, for_update=True), new_min, new_max)
         if rating_repository.get_by_name(session, payload.grade_name) is not None:
             raise BadRequestError("该等级名称已存在")
 
@@ -217,7 +217,7 @@ def update_grade(
             raise NotFoundError("评级等级不存在")
 
         _ensure_no_overlap(
-            rating_repository.list_all(session), new_min, new_max, exclude_id=grade_id
+            rating_repository.list_all(session, for_update=True), new_min, new_max, exclude_id=grade_id
         )
         existed = rating_repository.get_by_name(session, payload.grade_name)
         if existed is not None and existed.id != grade_id: