feat(download): switch download buttons to direct source links

The token relay still stood between the button and the file: every click
issued a short-lived token URL, so downloads never reached the real
source address directly.

- Render direct anchors (target=_blank) to the raw source URL in the
  download box and the paid card, with inline extract-code buttons
- Retire the relay: delete Content\Download plus its init wiring, header
  query-var branch and SEO noindex branch, and drop the dead
  download-request flow from cms-download.js (cashier, scan-pay and
  error views stay)
- Keep the dl_token table and the download logs page as read-only
  history; unscheduling of the orphaned cleanup cron is preserved

src/Admin/Native/PayDownloadLogs.php

@@ -76,7 +76,7 @@ class PayDownloadLogs {
         $total = $result['total'];
         $pages = max(1, (int) ceil($total / $per_page));
 
-        echo '<p class="description">' . esc_html__('基于下载令牌表（aksrt_dl_token）。已使用记录会保留一段时间后清理；未使用且过期的令牌会被定期删除。', 'aksrt') . '</p>';
+        echo '<p class="description">' . esc_html__('基于下载令牌表（aksrt_dl_token）。下载已改为直连模式，不再签发新令牌，以下为历史记录。', 'aksrt') . '</p>';
 
         echo '<form method="get" style="margin:12px 0;">';
         echo '<input type="hidden" name="page" value="' . esc_attr(self::PAGE_SLUG) . '" />';