Add show_order.php redirect to receipt page

Legacy URL payfr.it/show_order.php?UUID=... was returning 404.
Added PHP redirect (301) to /receipt/index.php which has the actual receipt logic.

show_order.php

@@ -0,0 +1,16 @@
+<?php
+/**
+ * Legacy redirect: show_order.php → /receipt/
+ * Old links (payfr.it/show_order.php?UUID=...) now point to the PHP receipt page.
+ */
+
+$uuid = $_GET['UUID'] ?? '';
+$isAdminView = $_GET['is_admin_view'] ?? '0';
+
+$params = http_build_query([
+    'UUID' => $uuid,
+    'is_admin_view' => $isAdminView,
+]);
+
+header("Location: /receipt/index.php?{$params}", true, 301);
+exit;