fix(分组): 防止降级环并校验上下文分组

- 增加降级链路环检测并拦截配置

- 仅复用合法分组上下文并必要时回退查询

- 标注 GetByIDLite 轻量语义并补充测试

backend/internal/repository/group_repo.go

@@ -70,6 +70,7 @@ func (r *groupRepository) GetByID(ctx context.Context, id int64) (*service.Group
 }
 
 func (r *groupRepository) GetByIDLite(ctx context.Context, id int64) (*service.Group, error) {
+	// AccountCount is intentionally not loaded here; use GetByID when needed.
 	m, err := r.client.Group.Query().
 		Where(group.IDEQ(id)).
 		Only(ctx)