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/constants/sections/payment.js

@@ -6,15 +6,6 @@ export const paymentSection = {
   icon: renderFaIcon('fas fa-wallet'),
   route: '/payment',
   children: [
-    customPage({
-      key: 'payment-paid',
-      name: 'PaymentPaid',
-      label: '下载/付费内容中心',
-      route: '/payment/paid',
-      title: '下载/付费内容中心',
-      desc: '资源下载门槛、付费内容策略和人工交付说明。',
-      component: 'ResourceDownload',
-    }),
     schemaPage({
       key: 'payment-recharge',
       name: 'PaymentRecharge',