Start SCA factor enrollment

Begin enrolling an SCA factor for the customer. Enrollment covers the explicit, opt-in factors a customer chooses to add — the request body's type selects TOTP or PASSKEY. Returns the factor-specific material needed to finish via POST /sca/factors/confirm.

SMS_OTP is implicit and is not enrolled through this endpoint. Every customer in an SCA-regulated region has a verified phone number from customer creation (via the Contact Verification flows — POST /customers/{customerId}/verify-phone and .../verify-phone/confirm), so SMS is always available as a factor with no extra setup and appears among the customer's enrolled factors in GET /sca/factors.

A customer may have only one passkey. Starting a passkey enrollment when one is already enrolled returns 409 (PASSKEY_ALREADY_ENROLLED) — delete it via DELETE /sca/factors/{credentialId} first.

This endpoint is only meaningful for customers in a region where SCA is required (e.g. EU). For customers outside SCA-regulated regions, this returns 409.