fix(langfuse): 테스트가 로컬 .env 의 LANGFUSE_HOST 로 진짜 전송하던 것 차단(autouse) + 게이트웨이 루트 span 에 질문/답 실음

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

5_django_backend/apps/gateway/views.py

@@ -102,7 +102,9 @@ def _observe(model_id: str, body: dict, col: _Collect, started: float, status: i
     u = col.usage or {}
     tid = str(uuid.uuid4())
     t0 = datetime.fromtimestamp(started, timezone.utc).isoformat()
-    root = langfuse.trace(tid, "fabrix", tags=["gateway"], metadata={"parts": kinds}, start=t0, end=langfuse.now_iso())
+    last_user = next((m.get("content") for m in reversed(body.get("messages") or []) if m.get("role") == "user"), None)
+    root = langfuse.trace(tid, "fabrix", tags=["gateway"], metadata={"parts": kinds}, start=t0, end=langfuse.now_iso(),
+                          input=last_user, output="".join(col.text))  # 목록에서 뭘 물었는지 바로 보이게
     langfuse.send_later([
         root,
         langfuse.generation(