Add pending_update field and scheduled_downgrade webhook for subscriptions

I want a `pending_update` field on the subscription object and a dedicated `subscription.scheduled_downgrade` webhook. This is important because it allows my app to natively handle and track scheduled downgrades (where a user keeps high-tier features until the next billing period) without having to infer changes from other webhooks or fields. This matches the functionality provided by Polar.sh's 'next_period' proration behavior.