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.

assets/admin/src/api/modules/permissions.js

@@ -10,7 +10,6 @@ export const permissionsApiMethods = {
   savePermissionsLoginSettings(data) { return this.saveSchemaPage('permissions', 'login', data) },
 
   getPermissionsModerationSettings() { return this.getSchemaPage('permissions', 'moderation') },
-  savePermissionsModerationSettings(data) { return this.saveSchemaPage('permissions', 'moderation', data) },
 
   getCapabilities() { return this.request('/permissions/capabilities') },
   saveCapabilities(user_caps) { return this.request('/permissions/capabilities', { method: 'POST', body: JSON.stringify({ user_caps }) }) },