Revert main to dev deployment model: remove bootstrap and use ../backend/ paths

Co-authored-by: Cursor <cursoragent@cursor.com>

public/mail-gateway.php

@@ -1,6 +1,6 @@
 <?php
-require_once __DIR__ . '/bootstrap.php';
-require_once HEXAHOST_CONFIG_DIR . '/products-config.php';
+require_once __DIR__ . '/../backend/includes/functions.php';
+require_once __DIR__ . '/../backend/config/products-config.php';
 
 // Produkt-Daten aus Config laden
 $product = getProduct('mail-gateway');