Initiates closure of a customer account. Closure runs asynchronously: a successful call starts the offboarding process and returns the account with its current status (still opened); the status transitions to closed once offboarding completes.

If the account has open blockers (active positions, a remaining cash balance, or overdue fees), it remains opened with offboarding checks in action_required and does not close until those are resolved. The outstanding actions are available as offboarding checks on the account.

The endpoint is idempotent: calling it again while a closure is in progress, or on an already closed account, returns 200 without starting a new closure.