Complete /v1/models schema in the OpenAPI contract

The published Model schema now declares every field GET /v1/models actually serves, not just the OpenAI-compatible core.

The Model schema previously declared only id, aliases, object, provider, model, api, and routable; the endpoint has served more for some time.

Newly declared: created and owned_by (OpenAI-compatibility fields), quality with its stage-fixed qualityUnit (% WER for stt, score for llm, MOS for tts), latencyMs, costPerMinUsd, and the per-model measured languages array.

Also declared: the TTS-only voices roster ({ id, name, gender, styles }) and its voicesVerified curation date, marked as present only on api: tts entries.

costPerMinUsd is documented with its real stage-dependent units: USD per minute for stt, per 1M tokens for llm, per 1M characters for tts.

All newly declared fields are optional in the schema, so responses from routers deployed before the voice-roster release continue to validate.

Contract documentation only; no response bytes or routing behavior change.