Expose new esperanto providers in the matrix: PPQ, Novita, Deepgram STT, Cohere

[lfnovo]

Part of the esperanto 2.25.1 adoption umbrella #1166.

Decision: support all the new providers, priority-ordered

esperanto now ships these upstream; we'll wire all of them into the Settings provider matrix, but land the mainstream ones first and the niche gateways lower on the list.

Higher priority (mainstream, first-class):

• Cohere (esperanto#110, 2.23.0) — LLM + embedding via native v2 API (COHERE_API_KEY). Not OpenAI-compatible: native /v2/chat + /v2/embed, with input_type-aware embeddings. (Cohere reranker is tracked separately under [Feature]: Support Rerankers #1087.)
• Deepgram STT (esperanto#202, 2.24.0) — complements the existing Deepgram TTS provider. DEEPGRAM_API_KEY, nova-3.

Lower priority (niche pay-as-you-go gateways):

• PayPerQ / PPQ (esperanto#238, 2.25.0) — multi-modality gateway (LLM + embedding + STT + TTS). Base URL https://api.ppq.ai/v1, PPQ_API_KEY.
• Novita (esperanto#120, 2.25.0) — pay-as-you-go open-source model gateway (LLM). NOVITA_API_KEY, base URL https://api.novita.ai/openai.

oMLX is a separate niche gateway already tracked at #1048 (PR #1164) — not duplicated here. OpenRouter TTS/STT (esperanto#224) is tracked at #987.

Task

For each provider, follow the existing provider-registry pattern: credential fields, model discovery, connection test, model registration, docs. No architectural decisions — esperanto already implements each; this is pattern-following exposure.

Acceptance

• Each provider selectable in Settings with working discovery + connection test.
• Docs updated per provider; smoke-tested with real keys where available.
• Cohere's native (non-OpenAI-compatible) surface verified end to end.