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/schema/sections/permissions/index.js

@@ -120,7 +120,7 @@ export const permissionsSchemas = {
    */
   moderation: {
     key: 'permissions.moderation',
-    title: '内容审核',
+    title: '评论审核',
     icon: 'fas fa-gavel',
     description: '管理评论审核、反垃圾、限流与拦截配置。',
     defaults: {