Confirms the booking at a concrete appointment time. The booking moves to Confirmed
with scheduledFor set, any outstanding proposal is cleared, and the member is
notified by email/SMS according to their notification preferences.

Member-agreement attestation: when the booking is currently RescheduleProposed,
the proposed time is still awaiting the member's acceptance in the app. Confirming in
that state moves the appointment without the member accepting through the platform, so
you must attest that the member agreed out-of-band (for example over the phone) by
sending memberAgreed: true — otherwise the request fails with 409
member_agreement_required. In other confirmable statuses memberAgreed is not
required.

A booking in the legacy Completed status cannot be confirmed (409 invalid_status).
The action is recorded in your activity log.