Expire credit grants on subscription renewal instead of a fixed number of days

Credit expiry is currently a fixed day count (30, 31, etc.) stamped when the grant is issued. Since calendar months vary between 28 and 31 days, it never lines up with a monthly billing cycle:

• 30 days: on a 31 day month credits expire a day early, so the customer is billed overage they should not owe
• 31 days: on shorter months unused credits carry into the next cycle

No setting avoids both, so merchants have to pick which error to live with.

Requested: an option for credits to expire on subscription renewal, so outstanding credits are cancelled and the new allocation issued at the same moment. Expiry then always matches the billing cycle regardless of month length.

Reported by: Eran Kutner, Port8 AI.