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/Schema/SchemaPermissions.php

@@ -74,7 +74,7 @@ class SchemaPermissions {
                 ],
                 'moderation' => [
                     'key'        => 'moderation',
-                    'title'      => '内容审核',
+                    'title'      => '评论审核',
                     'version'    => '1.0.0',
                     'option_key' => 'aksrt_moderation_config',
                     'groups'     => [