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/payment/index.js

@@ -2,7 +2,6 @@
  * Payment & Paid domain schemas.
  */
 
-import { paidSchema } from './paid.js'
 import { rechargeSchema } from './recharge.js'
 import { vipSchema } from './vip.js'
 import { revenueSchema } from './revenue.js'
@@ -13,7 +12,6 @@ import { cardPassSchema } from './cardPass.js'
 import { withdrawalSchema } from './withdrawal.js'
 
 export const paymentSchemas = {
-  paid: paidSchema,
   recharge: rechargeSchema,
   vip: vipSchema,
   revenue: revenueSchema,