refactor: 彻底移除 /posts/new 旧编辑器，统一迁移至 /studio/write

- 删除 /posts/new 路由，不再保留任何兼容层
- /posts/:id/edit 改为 301 跳转 /studio/write?id=:id
- 删除 posts/new.html 模板和 editor.js
- 移除 PostController.EditPage/Create/Update/Delete 孤儿方法
- 移除 /api/posts POST/PUT/DELETE 孤儿 API（保留 submit 和 upload-image）
- posts/show.html 编辑链接指向 /studio/write

templates/MetaLab-2026/html/posts/show.html

@@ -34,7 +34,7 @@
     {{if .IsLoggedIn}}
     <div class="post-actions">
         {{if and $.Post (or (eq $.Post.UserID $.UID) $.CanAccessAdmin) (ne $.Post.Status "pending") (not $.Post.IsLocked)}}
-        <a href="/posts/{{$.Post.ID}}/edit" class="btn btn-secondary">编辑</a>
+        <a href="/studio/write?id={{$.Post.ID}}" class="btn btn-secondary">编辑</a>
         {{end}}
         {{if and $.Post (eq $.Post.UserID $.UID) (or (eq $.Post.Status "draft") (eq $.Post.Status "rejected"))}}
         <button class="btn btn-primary" id="submitAuditBtn" data-id="{{$.Post.ID}}">提交审核</button>