- NOUS_BILLING_MANAGE_SCOPE constant.
- nous_token_has_billing_scope(): split-based scope check (no false-positive
substring match).
- step_up_nous_billing_scope(): re-runs the device flow requesting
billing:manage, reusing the held credential's portal/inference URLs + client_id
(so a preview stays a preview), persists like _login_nous but WITHOUT the model
picker. Returns True iff the minted token carries the scope (False when NAS
silently downscopes a non-admin / unticked grant).
Lazy step-up (plan D-A): normal login path unchanged; 403 insufficient_scope
from a billing call triggers this. 7 unit tests.
