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/routers/chat.py

@@ -18,14 +18,14 @@ import event_bus
 router = APIRouter()
 
 
-def _backend_label(backend: str, username: str) -> str:
+def _backend_label(backend: str, username: str, role: str = "chat") -> str:
     """Human-readable label for the model that handled a request."""
     if backend == "claude":
         return "Claude"
     if backend == "gemini":
         return "Gemini"
     if backend == "local":
-        cfg = model_registry.get_best_local_model(username)
+        cfg = model_registry.get_best_local_model(username, role)
         if cfg:
             return cfg.get("label") or cfg.get("model_name") or "Local"
         return "Local"
@@ -113,14 +113,16 @@ async def _stream_chat(req: ChatRequest):
             if not req.off_record:
                 log_turn(session_id, req.message, response_text)
 
-            requested = req.model or settings.primary_backend
+            # fallback_used only makes sense for explicit backend selections.
+            # In auto mode (req.model is None), just report what responded.
+            fallback_used = bool(req.model and actual_backend != req.model)
             payload = {
                 "type": "response",
                 "response": response_text,
                 "session_id": session_id,
                 "backend": actual_backend,
-                "backend_label": _backend_label(actual_backend, user),
-                "fallback_used": actual_backend != requested,
+                "backend_label": _backend_label(actual_backend, user, role="chat"),
+                "fallback_used": fallback_used,
             }
             yield f"data: {json.dumps(payload)}\n\n"