refactor(admin): clean up audit/permission/paid content config

- P1: remove the duplicate "审核联动" (ai_moderate_background/submit)
  block from the attachment permissions page; those settings live in
  the "评论审核" / API content audit surfaces.
- P2: rename the "内容审核" submenu to "评论审核" (permissions domain).
- P4: remove the "下载/付费内容中心" page and its schema/route chain;
  hardcode runtime policy so download always supports credit purchase
  and paid content is limited to posts. Drop the now-dead
  cms_features.download_credit flag and parse_slug_list helper.

src/Settings/HomeCms.php

@@ -137,7 +137,6 @@ class HomeCms {
     public static function get_cms_features(): array {
         $defaults = [
             'resource_cpt_enabled'    => true,
-            'download_credit'         => true,
             'search_include_resource' => true,
         ];
         $s = Settings::get_all();