Add customer.subscription.deleted webhook handler for auto-deactivation

[markgoho]

Context

The new "Cancel Membership" feature (replacing "Deactivate Membership") schedules Stripe subscription cancellation at the end of the billing period via cancel_at_period_end: true. However, when the period actually ends and Stripe fires the customer.subscription.deleted event, we don't yet handle it.

What needs to happen

When customer.subscription.deleted fires (billing period has ended after cancellation):

1. Deactivate member: Set membershipActive: false in Firestore
2. Draft profile: Set the member's Hugo profile to draft so it's hidden from the directory
3. Unsubscribe from newsletter: Remove from MailerLite subscriber list

Technical details

• The webhook handler should be added to the existing Stripe webhook processing pipeline
• Follow the same patterns as the existing charge.refunded handler
• The handler should be idempotent (safe to retry)
• Should send admin notification on failure

Related

• Cancel membership implementation: the PR that introduces cancel_at_period_end behavior