Support stacking multiple discounts on a single subscription

Currently only one discount can be active on a subscription at a time. When multiple discounts need to coexist (e.g., a loyalty discount + a retention offer), the only workaround is to manually calculate a combined value and replace the original discount.

For example, if a subscription is $100/mo with a 25% discount applied ($75/mo), and a 50% retention discount needs to be added on top, there's no way to stack them. The existing discount has to be replaced with a pre-calculated 62.5% combined discount. If either individual discount needs to change later, the combined value has to be recalculated and a new discount created, with no way to track the individual components.

It would be helpful to support multiple independent discounts on a single subscription, where each can be added, removed, or updated without affecting the others.