✓ Column 'midtrans_response' already exists.
✓ Column 'qr_created_at' already exists.

Current billings table schema:
- id (int(11))
- user_id (int(11))
- billing_code (varchar(50))
- amount (int(11))
- status (enum('waiting','paid','cancel'))
- midtrans_response (longtext)
- qr_created_at (datetime)
- created_at (timestamp)