Preview the upcoming invoice for a subscription

Computes what the customer's next invoice will look like, including any usage charges accumulated so far in the current billing period. Read-only — no charges, invoices, or line items are persisted.

Tax is computed using the same pipeline as the real invoice generator, so STRIPE_TAX merchants get accurate previews (at the cost of a tiny Stripe Tax usage call).

Returns 409 NOT_BILLABLE when the subscription is paused, cancelled, or already past its period end.