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/pages/payment/PaymentGateway.vue

@@ -7,7 +7,6 @@
     :links="[
       { label: '余额充值', path: '/payment/recharge' },
       { label: '充值卡', path: '/payment/card-pass' },
-      { label: '付费内容', path: '/payment/paid' },
     ]"
   >
     <template #before-form>