Returns the total amount currently held on the authenticated account: every
freeze placed on it, less everything since released.

Frozen money has already been deducted from the balance GET /api/balance
reports, so the two together are what the account held before any freeze. An
account with no holds returns 0.

This is a single figure covering every reason money can be held, including
holds placed while a Pix infraction (MED) is being worked. A hold can
therefore appear here before any notification explaining it arrives.