Subscription box manager

Build a monthly subscription box product with recurring billing.

Core features:
1. Plan selection (monthly/quarterly), Stripe subscription
2. Customer portal: skip a month, swap items, cancel
3. Fulfillment view for the ops team: this month's shipping list, addresses, sizes
4. Churn dashboard

Data: subscribers, plans, shipments, address_changes.
Must-haves: webhook to sync Stripe status, pause/resume, dunning emails.