大量消息收件箱完成第一层性能优化

src/Jiaowu.Api/Controllers/NotificationsController.cs

@@ -58,6 +58,7 @@ public sealed class NotificationsController(
         var total = await source.CountAsync(cancellationToken);
         var items = await source
             .OrderByDescending(x => x.CreatedAt)
+            .ThenByDescending(x => x.Id)
             .Skip((page - 1) * pageSize)
             .Take(pageSize)
             .Select(x => new