Subscription refunds and credits

Design the refund and credit handling for a subscription-billing platform. When a customer downgrades, cancels mid-cycle, or is overcharged, the system must compute prorated credits, decide whether to refund to the original payment method or issue account credit (a credit note applied to future invoices), and stay consistent with the revenue ledger. ~1M subscriptions, lots of plan changes daily. Describe the model and how you keep proration, refunds, credits, and recognized revenue consistent.