List Models

Return model list for client/gateway discovery. Plan-aware: keys entitled to Opus 4.7/4.8 (pro/ultra/payg) get 4.7+4.8 advertised FIRST so auto-discovery gateways default to them (PAYG = ví token, chỉ 4.7/4.8). Other keys (standard/lite/no-key) see the 4.6 base list — 4.7/4.8 ẩn để gateway không tự chọn rồi dính 403.