Schedule downgrades to take effect at the end of the billing period

Today all update_behavior options (proration-charge-immediately, proration-charge, proration-none) change the subscription immediately. There's no way to schedule a change for the next renewal, so the common SaaS pattern — upgrade immediately, downgrade at period end so the customer keeps what they paid for — has to be built in the merchant's own app: store a pending plan, keep serving the old tier, and call subscriptions.update shortly before current_period_end. Requested by two merchants in the Discord. Polar supports this natively as "Apply on next period."