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.

README.md

@@ -467,6 +467,46 @@ Default URLs:
   <p><i>...and 20+ more providers including Nebius, Chutes, Hyperbolic, and custom OpenAI/Anthropic compatible endpoints</i></p>
 </div>
 
+### 🏠 Self-hosted Providers
+
+For speech and embeddings served from **your own** machine — whisper.cpp,
+faster-whisper, Speaches, Kokoro-FastAPI, openedai-speech, llama.cpp/llama-server,
+vLLM, Infinity, text-embeddings-inference, or anything else that speaks the OpenAI
+shape.
+
+| Provider | Endpoint used | Typical server |
+| --- | --- | --- |
+| **Self-hosted STT** | `/v1/audio/transcriptions` | whisper.cpp, faster-whisper |
+| **Self-hosted TTS** | `/v1/audio/speech` | Kokoro-FastAPI, openedai-speech |
+| **Self-hosted Embedding** | `/v1/embeddings` | llama-server, vLLM, Infinity |
+
+Every other speech provider is a named cloud service with a fixed endpoint. These
+three read their address from **each connection**, so one provider can front
+several machines and load-balance across them like any other.
+
+Set it on the connection as `providerSpecificData.baseUrl`:
+
+| Provider | Give it | Result |
+| --- | --- | --- |
+| Self-hosted STT | the full URL — `http://host:8080/v1/audio/transcriptions` | used as-is |
+| Self-hosted TTS | the server root — `http://host:8880` | `+ /v1/audio/speech` |
+| Self-hosted Embedding | the **OpenAI base**, `/v1` included — `http://host:8080/v1` | `+ /embeddings` |
+
+> **Mind the `/v1` on embeddings.** The adapter appends `/embeddings`, so
+> `http://host:8080` resolves to `http://host:8080/embeddings` and misses the
+> OpenAI route — llama-server answers **501**. Give it the same base URL an OpenAI
+> client would use. A full `.../v1/embeddings` is also accepted, so a value pasted
+> from a `curl` example works too.
+
+The API key is not checked by most local servers, but the field must be non-empty:
+it is what gives the connection a credentials record, and `baseUrl` lives there.
+Any placeholder works.
+
+Self-hosted Embedding has **no cloud fallback by design** — a connection saved
+without a `baseUrl` is reported as a configuration error rather than quietly
+falling back to `api.openai.com`, which would send your input text and API key to
+a third party under a provider named "Self-hosted".
+
 ---
 
 ## 💡 Key Features