feat: MI50 as a Lyra backend (OpenAI-compatible local GPU)

The MI50 box (CT202) runs an OpenAI-compatible llama.cpp server on
10.0.0.44:8080. Wire it in as a third backend:

- llm.complete gains backend="mi50" (OpenAI client pointed at MI50_BASE_URL)
- config: MI50_BASE_URL (default http://10.0.0.44:8080/v1) + MI50_MODEL
- chat.respond labels the model per backend; web _backend_for maps "mi50"
- UI backend selector adds "MI50 — local GPU"

Verified end-to-end: llm.complete(backend="mi50") returns from the live server.
See homelab-inference memory for the box topology.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/llm.py

@@ -14,7 +14,7 @@ class Message(TypedDict):
     content: str
 
 
-Backend = Literal["local", "cloud"]
+Backend = Literal["local", "cloud", "mi50"]
 
 
 def complete(messages: list[Message], backend: Backend = "local") -> str:
@@ -26,6 +26,12 @@ def complete(messages: list[Message], backend: Backend = "local") -> str:
         resp = client.chat.completions.create(model=cfg.cloud_model, messages=messages)
         return resp.choices[0].message.content or ""
 
+    if backend == "mi50":
+        # MI50 box runs an OpenAI-compatible llama.cpp server; key is unused.
+        client = OpenAI(api_key="not-needed", base_url=cfg.mi50_base_url)
+        resp = client.chat.completions.create(model=cfg.mi50_model, messages=messages)
+        return resp.choices[0].message.content or ""
+
     resp = httpx.post(
         f"{cfg.local_base_url}/api/chat",
         json={"model": cfg.local_model, "messages": messages, "stream": False},