List Models
List available models with per-user discounted pricing when applicable.
Only models registered in the `models` table with is_enabled=true are
returned. Accepts either a dashboard JWT or an API key.
Includes pricing per 1 000 tokens and a convenience ``is_free`` flag.
If the caller has an active discount, discounted prices are included.
Response is cached for 5 minutes; admin writes invalidate the cache immediately.