feat: 新增店铺联级查询接口

- 新增 GET /api/admin/shops/cascade 接口，支持按店铺名模糊查询和上级ID过滤
- Store 层新增 ListForCascade（支持数据权限过滤）和 GetParentIDsWithChildren 方法
- Service 层新增 ListCascade，批量判断 has_children 避免 N+1 查询
- 返回格式：[{id, shop_name, has_children}]

💘 Generated with Crush

Assisted-by: Claude Sonnet 4.6 via Crush <crush@charm.land>

internal/store/postgres/shop_store.go

@@ -228,3 +228,40 @@ func (s *ShopStore) GetByIDs(ctx context.Context, ids []uint) ([]*model.Shop, er
 	}
 	return shops, nil
 }
+
+// ListForCascade 联级查询店铺（支持按名称模糊查询和按上级ID过滤）
+func (s *ShopStore) ListForCascade(ctx context.Context, shopName string, parentID *uint) ([]*model.Shop, error) {
+	query := s.db.WithContext(ctx).Model(&model.Shop{})
+	query = middleware.ApplyShopIDFilter(ctx, query)
+
+	if parentID != nil {
+		query = query.Where("parent_id = ?", *parentID)
+	} else {
+		query = query.Where("parent_id IS NULL")
+	}
+
+	if shopName != "" {
+		query = query.Where("shop_name LIKE ?", "%"+shopName+"%")
+	}
+
+	var shops []*model.Shop
+	if err := query.Order("created_at DESC").Find(&shops).Error; err != nil {
+		return nil, err
+	}
+	return shops, nil
+}
+
+// GetParentIDsWithChildren 查询指定 ID 列表中哪些店铺有下级（返回作为上级的店铺ID集合）
+func (s *ShopStore) GetParentIDsWithChildren(ctx context.Context, ids []uint) ([]uint, error) {
+	if len(ids) == 0 {
+		return []uint{}, nil
+	}
+	var parentIDs []uint
+	if err := s.db.WithContext(ctx).Model(&model.Shop{}).
+		Distinct("parent_id").
+		Where("parent_id IN ?", ids).
+		Pluck("parent_id", &parentIDs).Error; err != nil {
+		return nil, err
+	}
+	return parentIDs, nil
+}