Apply discounts to existing subscriptions

[pieterbeulque]

When a "once" discount is applied to an existing active subscription mid-billing-cycle, the discount is immediately considered expired and will never be applied to any invoice.

# server/polar/models/discount.py:144-147

def is_repetition_expired(self, started_at, current_period_start, trial_ended=False):
    if self.duration == DiscountDuration.once:
        return not trial_ended

For repeating discounts, it's influenced by the subscription's start date too, so it also won't work as expected. Applying a 3 month 50% off discount in the fifth month of the subscription will just consider the discount expired.

You'd expect it to apply to the next invoice, which is useful for cases like customer support granting next month for free as compensation for an issue, or trying to prevent churn with next X charges 80% off.

[pieterbeulque]

Fixed in #8826 and #8827