When contact-config.php exists but is unreadable by PHP-FPM (e.g. owned by
www-data while pool runs as nginx), file_exists() returns true but
require_once throws a fatal error → 500.
Production was hitting this: contact-config.php was 'www-data:www-data 640'
but the sag24 PHP-FPM pool runs as 'nginx'. Form returned HTTP 500 on every
POST including legitimate ones — contact form effectively dead.
Server side: chown nginx:nginx /opt/www/sag24.ru/contact-config.php (already
applied). This commit makes the script defensive against the same scenario
in the future.