refactor(admin): remove download config settings page and chain

- Drop the download config menu entry, route and schema from the settings SPA
- Remove the PHP schema section, the server-side alias validation and the
  reserved-alias list that only served that section
- Unregister aksrt_download_config from standalone import/export keys
- Drop the contract tests that locked the removed page; the Download.php
  runtime contracts stay untouched

assets/admin/src/constants/sections/permissions.js

@@ -47,15 +47,5 @@ export const permissionsSection = {
       canonicalDomain: 'permissions',
       section: 'attachment',
     }),
-    schemaPage({
-      key: 'permissions-download',
-      name: 'PermissionsDownload',
-      label: '下载配置',
-      route: '/permissions/download',
-      title: '下载配置',
-      desc: '下载链接别名、默认使用次数与有效期限规则。',
-      canonicalDomain: 'permissions',
-      section: 'download',
-    }),
   ],
 }

assets/admin/src/schema/sections/permissions/index.js

@@ -338,41 +338,4 @@ export const permissionsSchemas = {
   ],
 },
 
-  /**
-   * Download link configuration
-   */
-  download: {
-    key: 'permissions.download',
-    title: '下载配置',
-    icon: 'fa fas fa-download',
-    description: '下载总开关与链接别名、默认使用次数、有效期限规则。',
-    defaults: {
-      enabled: true,
-      url_alias: 'download',
-      default_max_uses: 1,
-      ttl_minutes: 10,
-    },
-    groups: [
-      {
-        key: 'basic',
-        title: '下载总开关',
-        icon: 'fa fas fa-power-off',
-        fields: [
-          { key: 'enabled', label: '启用本站下载链接', type: 'switch', help: '总开关：开启后下载通过本站中转链接；关闭后下载按钮直接使用原始下载地址（直连），下方链接规则全部不生效，旧的中转链接同步失效。' },
-        ],
-      },
-      {
-        key: 'rules',
-        title: '链接规则',
-        icon: 'fa fas fa-link',
-        description: '文件完整送达才计次（HEAD 探测、传输中断不计次）；到期或耗尽后访问旧地址返回 410，需重新获取，不会自动续签。',
-        fields: [
-          { key: 'url_alias', label: '下载链接 URL 别名', type: 'input', placeholder: 'download', required: true, dependsOn: { key: 'enabled', value: true }, help: '仅允许英文字母、数字、连字符和下划线；禁止中文、空格和 / \\ ? # & 等特殊符号。修改后旧别名路由立即失效。' },
-          { key: 'default_max_uses', label: '下载链接默认使用次数', type: 'number', min: 1, max: 100, dependsOn: { key: 'enabled', value: true }, help: '单个链接可完成下载的次数；耗尽后链接返回 410，需重新获取。' },
-          { key: 'ttl_minutes', label: '下载链接默认有效期限（分钟）', type: 'number', min: 1, max: 1440, dependsOn: { key: 'enabled', value: true }, help: '链接签发后超过该时长即失效并返回 410，不自动续期。' },
-        ],
-      },
-    ],
-  },
-
 };