feat(paykit): add customer API methods and rename plans to subscriptions

- Add upsertCustomer, deleteCustomer, listCustomers to PayKit instance
- Replace ensureCustomer with upsertCustomer across codebase
- Hard delete customers with full Stripe cleanup (cancel subs, delete customer)
- listCustomers returns paginated customers with subscriptions and entitlements
- Rename event payload field from plans to subscriptions
- Add deleteCustomer and listActiveSubscriptions to StripeRuntime