BuyupWebhookSyncListener

Listens to normalized Stripe subscription webhook routing actions and reconciles Benecaster’s stored buy-up grants against Stripe’s subscription item state. Registered at priority 20 on MembershipWebhookHandlers::ACTION_SUBSCRIPTION_UPDATED and ACTION_SUBSCRIPTION_DELETED, running after Benecaster built-in membership handlers at priority 10. On subscription update, syncs new buy-up grants and marks cancelling items. On subscription deletion, terminates all active buy-up grants for the subscription.