Silence REQUEST_METHOD warning when bootstrap runs from the CLI

The migration and backfill scripts load bootstrap.php outside a request, where
$_SERVER['REQUEST_METHOD'] does not exist, so every CLI run printed a warning
before its output.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/core/bootstrap.php

@@ -51,7 +51,8 @@ if (in_array($origin, $allowedOrigins)) {
 header('Access-Control-Allow-Methods: POST, GET, OPTIONS');
 header('Access-Control-Allow-Headers: Content-Type, Authorization, X-Device-FP, X-HMAC-Auth, X-Internal-Key');
 
-if ($_SERVER['REQUEST_METHOD'] === 'OPTIONS') {
+// REQUEST_METHOD غير معرّف عند التشغيل من سطر الأوامر (سكربتات الترحيل)
+if (($_SERVER['REQUEST_METHOD'] ?? '') === 'OPTIONS') {
     http_response_code(200);
     exit;
 }