Auth Impersonate Switch Org Partial Update

Endpoint for switching organization while impersonating a user. Security: gated on the ``staff_impersonate`` session scope. Operators revoke per-user via ``APIUser.has_impersonation_access`` — see ``ImpersonateUserView`` for the model. Switching organizations is a mid-session operation that only makes sense WHILE impersonating, so the same scope gates it (no separate switch scope).