feat(admin): 用户管理新增分组列、分组筛选与专属分组一键替换

- 新增分组列：展示用户的专属/公开分组，支持 hover 查看详情
- 新增分组筛选：下拉选择或模糊搜索分组名过滤用户
- 专属分组替换：点击专属分组弹出操作菜单，选择目标分组后
  自动授予新分组权限、迁移绑定的 Key、移除旧分组权限
- 后端新增 POST /admin/users/:id/replace-group 端点，事务内
  完成分组替换并失效认证缓存

backend/internal/handler/sora_client_handler_test.go

@@ -942,6 +942,9 @@ func (r *stubUserRepoForHandler) ExistsByEmail(context.Context, string) (bool, e
 func (r *stubUserRepoForHandler) RemoveGroupFromAllowedGroups(context.Context, int64) (int64, error) {
 	return 0, nil
 }
+func (r *stubUserRepoForHandler) RemoveGroupFromUserAllowedGroups(context.Context, int64, int64) error {
+	return nil
+}
 func (r *stubUserRepoForHandler) UpdateTotpSecret(context.Context, int64, *string) error { return nil }
 func (r *stubUserRepoForHandler) EnableTotp(context.Context, int64) error                { return nil }
 func (r *stubUserRepoForHandler) DisableTotp(context.Context, int64) error               { return nil }
@@ -1017,6 +1020,20 @@ func (r *stubAPIKeyRepoForHandler) SearchAPIKeys(context.Context, int64, string,
 func (r *stubAPIKeyRepoForHandler) ClearGroupIDByGroupID(context.Context, int64) (int64, error) {
 	return 0, nil
 }
+func (r *stubAPIKeyRepoForHandler) UpdateGroupIDByUserAndGroup(_ context.Context, userID, oldGroupID, newGroupID int64) (int64, error) {
+	var updated int64
+	for id, key := range r.keys {
+		if key.UserID != userID || key.GroupID == nil || *key.GroupID != oldGroupID {
+			continue
+		}
+		clone := *key
+		gid := newGroupID
+		clone.GroupID = &gid
+		r.keys[id] = &clone
+		updated++
+	}
+	return updated, nil
+}
 func (r *stubAPIKeyRepoForHandler) CountByGroupID(context.Context, int64) (int64, error) {
 	return 0, nil
 }