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).