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/Core/SEOHead.php

@@ -26,8 +26,6 @@ class SEOHead {
         }
         // 单一出口：所有 SEO/OG/JSON-LD 输出集中在 priority 2。
         add_action('wp_head', [$this, 'render'], 2);
-        // robots：dl token 页面 noindex（独立、最早输出）。
-        add_action('wp_head', [$this, 'render_robots'], 1);
     }
 
     /**
@@ -229,15 +227,6 @@ class SEOHead {
         return '';
     }
 
-    /**
-     * dl token 页面 noindex。
-     */
-    public function render_robots(): void {
-        if (function_exists('get_query_var') && get_query_var('aksrt_dl_token')) {
-            echo '<meta name="robots" content="noindex,nofollow">' . "\n";
-        }
-    }
-
     /**
      * 输出 head（favicon + canonical + description/keywords + OG/Twitter + JSON-LD）。
      */