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/providers/registry/selfhosted-tts.js

@@ -0,0 +1,44 @@
+// Self-hosted, OpenAI-compatible text-to-speech (Kokoro-FastAPI, openedai-speech,
+// vLLM-served TTS, ...) — the TTS counterpart of selfhosted-stt.
+//
+// Every other self-hostable TTS provider here (coqui, tortoise) carries a FIXED
+// localhost baseUrl in its registry entry and `authType: "none"`, and the generic
+// dispatcher reads `ttsConfig.baseUrl` from that entry rather than from the
+// connection. So there was no way to point TTS at a server on another host.
+//
+// `authType: "apikey"` is what makes the override possible at all: it gives the
+// connection a credentials record, which is where providerSpecificData.baseUrl
+// lives. Local servers ignore the key; any non-empty value works.
+export default {
+  id: "selfhosted-tts",
+  priority: 50,
+  hasFree: true,
+  alias: "selfhosted-tts",
+  display: {
+    name: "Self-hosted TTS",
+    icon: "cloud",
+    color: "#ffffffff",
+    textIcon: "TT",
+    website: "https://github.com/remsky/Kokoro-FastAPI",
+  },
+  category: "apikey",
+  auth: {
+    apiKey: {
+      text: "Set providerSpecificData.baseUrl to the server root, e.g. http://host:8080 — /v1/audio/speech is appended. The API key is not checked by local servers; any value works.",
+    },
+  },
+  // Voice is selected as "<model>/<voice>", the same convention the OpenAI TTS
+  // adapter uses, so existing clients need no special casing.
+  models: [
+    { id: "kokoro", name: "Kokoro (self-hosted)", params: ["voice", "response_format", "speed"], kind: "tts" },
+  ],
+  serviceKinds: ["tts"],
+  ttsConfig: {
+    // Overridden per connection by providerSpecificData.baseUrl; this default
+    // only makes the provider usable on a same-host deployment.
+    baseUrl: "http://localhost:8880",
+    defaultModel: "kokoro",
+    authType: "apikey",
+    format: "openai-speech",
+  },
+};