Update Brain Configuration

The customer-editable Brain fields. extra="forbid".

brain_live was the other field here. It is removed: the flag no longer exists anywhere downstream (pips always sets hints.decision_owner to brain on triggers it sends; some interactions stay on pips with no trigger), and the UI that toggled it is gone.

Because this model forbids extras, a client that still sends brain_live now gets a 422 rather than a silent no-op. That is the intended direction -- a save that cannot take effect should say so -- but it is why the UI had to stop sending it first.