Permissions required: Subaccount

Set the subaccount's enrollment in the fiat cash interest program.

The subaccount's enrollment state is updated before this call returns, and
the response reports it. Its fiat wallets are then aligned to that state in
the background, so they follow the 202 rather than coincide with it. If that
alignment proves impossible the stored state is rolled back, and a later read
reports the previous value: the state means interest is active on the
subaccount, not that a change to it was requested.

Only the fiat wallets the subaccount holds at the moment of the call are
aligned. A fiat wallet provisioned afterwards does not inherit the setting,
and re-sending this request does not correct it — a request matching the
subaccount's current state succeeds without touching any wallet. Aligning
wallets created later is not supported yet.

A subaccount created under an enrolled firm starts enrolled, and one created
under a firm that is not enrolled starts unenrolled; use this endpoint to
change that afterwards, in either direction. Enrolling is rejected with a 422
unless the firm itself has been granted access to the interest feature;
unenrolling is never gated.

A subaccount that is closed, or awaiting review, cannot be changed at all and
is rejected with a 409 whichever state is requested — including a request
that matches the state it already holds.