refactor: simplify environment variable loading with helper function and remove unused mail configuration

.env.example

@@ -18,8 +18,3 @@ MINIO_ENDPOINT=http://localhost:9000
 MINIO_BUCKET_NAME=default_bucket
 MINIO_ACCESS_KEY=your-minio-access-key
 MINIO_SECRET_KEY=your-minio-secret-key
-MAIL_USER=your-email@example.com
-MAIL_PASSWORD=your-smtp-password
-MAIL_HOST=smtp.gmail.com
-MAIL_PORT=465
-MAIL_SECURE=true