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) {

open-sse/handlers/ttsProviders/selfhostedTts.js

@@ -0,0 +1,69 @@
+// Self-hosted OpenAI-compatible TTS — POST {baseUrl}/v1/audio/speech.
+//
+// A SPECIAL_ADAPTER rather than a genericFormats handler on purpose: the generic
+// dispatcher resolves baseUrl from the static registry entry
+// (`synthesizeViaConfig` reads `cfg.baseUrl`) and never looks at the connection,
+// which is exactly the limitation this provider exists to lift.
+import { Buffer } from "node:buffer";
+
+const DEFAULT_BASE_URL = "http://localhost:8880";
+const DEFAULT_MODEL = "kokoro";
+const DEFAULT_VOICE = "af_heart";
+
+export default {
+  async synthesize(text, model, credentials, responseFormat = "mp3") {
+    // Accept either providerSpecificData.baseUrl (how the custom embedding and
+    // STT providers carry it) or a bare credentials.baseUrl (how the OpenAI TTS
+    // adapter does), so a connection configured either way works.
+    const raw = credentials?.providerSpecificData?.baseUrl || credentials?.baseUrl || DEFAULT_BASE_URL;
+    // Tolerate a baseUrl given as the full endpoint or with a trailing /v1 —
+    // both are natural things to paste, and silently double-appending the path
+    // would 404 with nothing pointing at the cause.
+    const base = String(raw)
+      .replace(/\/+$/, "")
+      .replace(/\/v1\/audio\/speech$/, "")
+      .replace(/\/v1$/, "");
+
+    // The provider prefix is already stripped by getModelInfo, so `model` here is
+    // "kokoro" or "kokoro/af_heart" — NOT "selfhosted-tts/...".
+    //
+    // A bare value is the MODEL, not the voice. The OpenAI adapter reads a bare
+    // value as a voice, which is right for a service whose model is fixed
+    // ("tts-1") and whose voice varies — but wrong here, where the model is the
+    // variable part. Treating it as a voice sent voice="kokoro" upstream and
+    // Kokoro answered 400, so `selfhosted-tts/kokoro` — the obvious way to
+    // address this provider — was the one form that did not work (verified
+    // against a live Kokoro through 9router, 2026-08-03).
+    let ttsModel = DEFAULT_MODEL;
+    let voice = DEFAULT_VOICE;
+    if (model) {
+      const parts = String(model).split("/").filter(Boolean);
+      if (parts.length >= 2) {
+        ttsModel = parts[0];
+        voice = parts.slice(1).join("/");
+      } else if (parts.length === 1) {
+        ttsModel = parts[0];
+      }
+    }
+
+    const res = await fetch(`${base}/v1/audio/speech`, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        ...(credentials?.apiKey ? { Authorization: `Bearer ${credentials.apiKey}` } : {}),
+      },
+      body: JSON.stringify({
+        model: ttsModel,
+        voice,
+        input: text,
+        response_format: responseFormat,
+      }),
+    });
+    if (!res.ok) {
+      const err = await res.json().catch(() => ({}));
+      throw new Error(err?.error?.message || `Self-hosted TTS failed: ${res.status}`);
+    }
+    const buf = await res.arrayBuffer();
+    return { base64: Buffer.from(buf).toString("base64"), format: responseFormat };
+  },
+};