feat(providers): self-hosted OpenAI-compatible STT, TTS and embedding providers

Add Self-hosted STT/TTS/Embedding providers that read baseUrl per connection
instead of a fixed registry endpoint, so 9Router can point at whisper.cpp,
faster-whisper, Kokoro-FastAPI, llama-server, vLLM, Infinity, and similar
OpenAI-compatible local servers.

Self-hosted Embedding refuses to run without a baseUrl rather than falling
back to api.openai.com like openaiCompatNode does, since that fallback would
silently send input text and the API key to OpenAI under a provider named
"Self-hosted". Also fixes embeddingsCore to catch adapter build errors as a
400 instead of letting them escape uncaught, and bounds the upstream fetch
with FETCH_CONNECT_TIMEOUT_MS to avoid hanging forever on a dead endpoint.

Self-hosted TTS treats a bare model value as the model rather than the voice,
since the generic OpenAI TTS convention (bare = voice) is backwards for a
provider where the model is the variable part.

open-sse/handlers/ttsProviders/index.js

@@ -7,6 +7,7 @@ import openai from "./openai.js";
 import openrouter from "./openrouter.js";
 import gemini, { fetchGeminiVoices } from "./gemini.js";
 import xiaomiMimo from "./xiaomi-mimo.js";
+import selfhostedTts from "./selfhostedTts.js";
 import { FORMAT_HANDLERS } from "./genericFormats.js";
 import { parseModelVoice } from "./_base.js";
 
@@ -20,6 +21,7 @@ const SPECIAL_ADAPTERS = {
   openrouter,
   gemini,
   "xiaomi-mimo": xiaomiMimo,
+  "selfhosted-tts": selfhostedTts,
 };
 
 export function getTtsAdapter(provider) {