游标分页优化

src/Jiaowu.Api/Contracts/PagedResult.cs

@@ -8,7 +8,8 @@ public sealed record CursorPagedResult<T>(
     IReadOnlyCollection<T> Items,
     string? NextCursor,
     bool HasMore,
-    int PageSize);
+    int PageSize,
+    int? Total = null);
 
 /// <summary>
 /// Legacy offset-page response. New list endpoints must use