Reviewed code and made several improvements which provide better error handling, code quality, and
resolved warnings. The changes include:
- Syntax modifications for better readability,
- Ensured variable type checks before using methods,
- Resolved warning instances in phpstan,
- Adapted code to comply with modern PHP nullable type indicator.

Alongside, made version update in phive/phars.xml. Also, unnecessary comments or instances are
removed for a cleaner code.

Related: #16

Class QUI\DataBase\Exception not found, variable $whereOr in empty() always exists and is not falsy

feat: coupon send via mail now optional; refactor: delete hidden discounts associated with coupons when deleted

refactor: reusable status changed to maxUsages with 3 different options