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/Admin/GlobalController.php

@@ -182,7 +182,6 @@ class GlobalController extends ControllerBase {
         $params = $this->get_json_params($request);
         $allowed = [
             'resource_cpt_enabled'    => 'bool',
-            'download_credit'         => 'bool',
             'search_include_resource' => 'bool',
         ];
         $sanitized = $this->sanitize_fields($params, $allowed);