feat: connect white paper download form to Brevo

Refactors the download route into clean single-responsibility functions
and adds Brevo contact sync (list ID configurable via brevo_whitepaper.list_id).
Includes a stub for future external email deliverability validation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

site/config/config.php

@@ -12,6 +12,11 @@ return [
 
     'thumbs' => require __DIR__ . '/thumbs.php',
 
+    'brevo_whitepaper' => [
+        'api_key' => '', // leave empty in dev to skip Brevo sync
+        'list_id' => 5,
+    ],
+
     'cache' => [
         'redirects' => ['type' => 'file'],
     ],