fix(ERP-246): add missing api1/views/site/index.php to fix ViewNotFoundException

fix(ERP-246): add missing api1/views/site/index.php to fix ViewNotFoundException

api1 and main app share namespace app\controllers, so SiteController
from the main app is accessible via api1. When GET / hits api1, Yii2
routes to SiteController::actionIndex() which calls render('index'),
but api1/views/site/index.php did not exist → ViewNotFoundException (500).

Added empty view file to stop the error. api1 is a REST API (FORMAT_JSON)
so the rendered empty string is JSON-encoded as "".

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