Subscribe a user to a plan and collect payment.

The subscription and installment are created immediately. Payment collection happens asynchronously
and does not affect the response. If paymentMethodId is provided, that payment method is used
exclusively. If omitted, the system uses automatic payment method resolution. For free pre-paid plans (price = 0) and post-paid plans, payment is skipped.

If the user already has an active subscription to a different plan, that subscription is cancelled
and the new one is activated.