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/web/server.py

@@ -32,7 +32,10 @@ _CLOUD = {"OPENAI", "cloud", "custom"}
 
 
 def _backend_for(label: str | None) -> Backend:
-    if label and label.upper() in {"PRIMARY", "SECONDARY", "FALLBACK", "LOCAL"}:
+    key = (label or "").lower()
+    if key == "mi50":
+        return "mi50"
+    if key in {"local", "primary", "secondary", "fallback"}:
         return "local"
     return "cloud"