Better lifecycle emails for web billing failures

RevenueCat Billing now sends three distinct lifecycle emails to handle payment failures. This replaces the single generic billing issue email that went out regardless of the subscriber's actual access status.

When a renewal fails, subscribers receive an email that matches their exact state. They get a grace period email if they still have access, a retry email during the retry window, and a specific expiration email if they lose access entirely.

This stops subscribers from receiving contradictory messages about their access. It reduces confusion when a payment is still retrying in the background, which cuts down on support tickets and refund requests from frustrated customers who thought they had already lost access. The messaging now accurately reflects the subscription state.