[RhysSullivan]

Adds what Executor's v1 import needs to move a paid customer between two Autumn catalogs in one account, modelled on what the real sandbox did (x-api-version 2.3.0):

• customers.update with new_customer_id. Subscriptions, Stripe subscription and usage stay. A taken id gets 409 customer_already_exists.
• billing.preview_attach, which takes the same body as attach and changes nothing.
• billing.attach with remove_plan_ids and proration_behavior: "none". The new plan takes over the removed paid plan's Stripe subscription with its period and a running trial. Nothing is charged and there is no checkout. A cancellation at the end of the cycle is dropped, and a scheduled plan stays scheduled. customize (flat price, items) and version are supported. Removing a running trial without proration gets 400, a missing plan gets 404 product_not_found, and attaching a plan already held gets 409.
• customers.get with expand: ["subscriptions.plan"] returns the version each subscription bills.
• billing.verify lists each paid plan's Stripe subscription.
• customers.delete.
• Seed config: a plan's versions, and a subscription's version and stripe_subscription_id.

Tests: src/__tests__/autumn-carry-over.test.ts. The package suite passes (69 tests).

This only adds behaviour. Existing routes are unchanged except that a checkout-settled paid plan now records a Stripe subscription id. Executor's tests PR for the billing carry-over can't pass until this is deployed to emulators.dev.