feat: OpenAI-compatible orchestrator + backend auto-routing

- openai_orchestrator.py — new ReAct tool loop engine for any
  OpenAI-compatible endpoint (OpenRouter, Open WebUI, Ollama, LiteLLM);
  model handles both tool loop and final response, no Claude handoff needed
- tools/__init__.py — auto-derive OpenAI JSON Schema from existing Gemini
  FunctionDeclarations so tool definitions have a single source of truth
- routers/orchestrator.py — route to openai_orchestrator when model registry
  "orchestrator" role resolves to a local_openai type host
- routers/chat.py — pass role to _backend_label(); fix fallback_used logic
  (only meaningful for explicit backend overrides, not auto-routing)
- static/app.js — add null/"auto" to backend cycle; fetch local model hint
  without overriding the auto default on page load
- model_registry.py — _normalize() back-fills host_type on old registry files
- requirements.txt — add openai>=1.0.0
- ARCH__BACKENDS.md — document OpenAI-compat backend and routing logic

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/model_registry.py

@@ -122,13 +122,20 @@ def _empty() -> dict:
     return {"version": 1, "hosts": [], "models": [], "roles": {}}
 
 
+def _normalize(data: dict) -> dict:
+    """Back-fill any missing fields introduced by schema additions."""
+    for h in data.get("hosts", []):
+        h.setdefault("host_type", "openwebui")
+    return data
+
+
 def _load(username: str) -> dict:
     path = _registry_path(username)
     if path.exists():
         try:
             data = json.loads(path.read_text())
             if isinstance(data, dict) and "version" in data:
-                return data
+                return _normalize(data)
         except (json.JSONDecodeError, OSError):
             logger.warning("model_registry.json for %s is unreadable — starting fresh", username)
         return _empty()