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/router/component-loaders/core.js

@@ -13,7 +13,6 @@ export const corePageLoaders = {
   PermissionsCapabilities: () => import('@/pages/permissions/PermissionsCapabilities.vue'),
   AttachmentPermissions: () => import('@/pages/permissions/AttachmentPermissions.vue'),
   CmsCredits: () => import('@/pages/CmsCredits.vue'),
-  ResourceDownload: () => import('@/pages/ResourceDownload.vue'),
   ThemeInfoPage: () => import('@/pages/theme/ThemeInfoPage.vue'),
   ThemeAuthorizationPage: () => import('@/pages/theme/ThemeAuthorizationPage.vue'),
   DocsPage: () => import('@/pages/docs/DocsPage.vue'),