Attach a Quickstart-managed plan to a customer

Creates a Stripe Subscription on the merchant's connected account for the given Drip (customer, plan) pair. The plan MUST be Drip-managed (managedBy=DRIP); run provisionPlanOnStripe / POST /pricing-plans/:id/reprovision-stripe first if the plan is still EXTERNAL. Idempotent: re-calling with the same customer + plan returns the existing subscription.

Requires a secret key (sk_*) with the ADMIN role.