Set customer-unit cap

Sets the cap on this customer unit for one credits currency. Returns changed: false without creating a version when the effective timeline is unchanged; otherwise records a new version, preserving earlier rows. The newest version wins where they overlap. effectiveFrom controls applicability; schedule defines monthly UTC boundaries independently. Omit schedule to inherit the version covering effectiveFrom, or use the creation-time UTC day if none applies. Select the currency with creditsCurrencyId in the body; it may be omitted only when the organization has exactly one credits currency. A cap covers the unit and every unit beneath it, so a cap on the customer's root unit covers every unit of that customer; spend from a signal that names no unit counts toward no cap. 404 when the customer or the unit does not exist. 409 for customers on seat-based billing, a deleted unit (CUSTOMER_UNIT_DELETED) or a draft (CUSTOMER_UNIT_DRAFT). Use the value returned as customer.id, for example cus_abc123; if you have your own customer ID, use the /api/v2/customers/external/{externalId}/… twin.