fix: [deploy] Google Storage credentials 심링크 추가

- 배포 시 shared/storage/credentials → storage/credentials 심링크 생성
- Google Cloud 서비스 계정 JSON 파일 접근 보장

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

Jenkinsfile

@@ -45,6 +45,7 @@ pipeline {
                             ln -sfn /home/webservice/mng/shared/.env .env &&
                             sudo chmod 640 /home/webservice/mng/shared/.env &&
                             ln -sfn /home/webservice/mng/shared/storage/app storage/app &&
+                            ln -sfn /home/webservice/mng/shared/storage/credentials storage/credentials &&
                             composer install --no-dev --optimize-autoloader --no-interaction &&
                             npm install --prefer-offline &&
                             npm run build &&