feat: 添加音乐键位字段，更新相关数据结构和接口

frontend/portal/src/views/creator/ContentsEditView.vue

@@ -242,6 +242,7 @@ const loadContent = async (id) => {
 
         form.genre = res.genre;
         form.title = res.title;
+        form.key = res.key;
         form.abstract = res.description;
         form.price = res.price;
         form.priceType = res.price > 0 ? 'paid' : 'free';
@@ -332,6 +333,7 @@ const submit = async () => {
          title: form.title,
          description: form.abstract,
          genre: form.genre,
+         key: form.key,
          status: 'published', // Direct publish for demo
          visibility: 'public',
          preview_seconds: form.enableTrial ? form.trialTime : 0,