feat(search): 添加封面关联查询以优化搜索结果展示

- 在 SubContent 查询中新增 cover 关联关系
- 为 cover 关联配置字段筛选 (id, name, url, type)
- 保持原有的 thumbnail 和 category 关联查询逻辑
- 优化搜索结果中媒体资源的加载效率

app/controller/frontend/SearchController.php

@@ -226,7 +226,9 @@ class SearchController extends BaseController
         $baseWhere = ['seller_id' => $this->sellerId, 'is_del' => 1];
         $content = (new SubContent())->with(['thumbnail'=>function($q){
             $q->field('id,name,url,type');
-        },'category'])->where($baseWhere)->whereIn('id', $listSubIds ?: [0]);
+        },'category', 'cover'=>function($q){
+            $q->field('id,name,url,type');
+        }])->where($baseWhere)->whereIn('id', $listSubIds ?: [0]);
 
         $productIdSet = array_flip($productContentIds);
         $newsIdSet = array_flip($newsContentIds);