feat: store and display backend + host metadata on chat messages

Each assistant message in the session JSON now carries:
  backend, backend_label, host (platform.node())

These fields are shown as model tags in the UI — on live responses and
when loading session history. Session log entries (sessions/YYYY-MM-DD.md)
include the backend label and host in the turn header.

The local (OpenAI-compat) backend strips non-standard fields before
sending messages to the API so extra fields don't leak upstream.

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

cortex/routers/chat.py

@@ -1,5 +1,6 @@
 import asyncio
 import json
+import platform
 import jwt
 from fastapi import APIRouter, HTTPException, Query, Request
 from fastapi.responses import StreamingResponse
@@ -108,10 +109,18 @@ async def _stream_chat(req: ChatRequest):
 
         try:
             response_text, actual_backend = task.result()
-            history.append({"role": "assistant", "content": response_text})
+            backend_label = _backend_label(actual_backend, user, role="chat")
+            host = platform.node()
+            history.append({
+                "role": "assistant",
+                "content": response_text,
+                "backend": actual_backend,
+                "backend_label": backend_label,
+                "host": host,
+            })
             save_session(session_id, history)
             if not req.off_record:
-                log_turn(session_id, req.message, response_text)
+                log_turn(session_id, req.message, response_text, backend_label, host)
 
             # fallback_used only makes sense for explicit backend selections.
             # In auto mode (req.model is None), just report what responded.
@@ -121,7 +130,8 @@ async def _stream_chat(req: ChatRequest):
                 "response": response_text,
                 "session_id": session_id,
                 "backend": actual_backend,
-                "backend_label": _backend_label(actual_backend, user, role="chat"),
+                "backend_label": backend_label,
+                "host": host,
                 "fallback_used": fallback_used,
             }
             yield f"data: {json.dumps(payload)}\n\n"