fix(langfuse): v4 가 옛 ingestion 을 거부 — OTLP/HTTP JSON(/api/public/otel/v1/traces)으로 전환, 로컬 Langfuse 실측 통과. 사용자·세션 속성은 자식 span 에도 복사

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

5_django_backend/apps/gateway/views.py

@@ -101,11 +101,13 @@ def _observe(model_id: str, body: dict, col: _Collect, started: float, status: i
         return
     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())
     langfuse.send_later([
-        langfuse.trace(tid, "fabrix", tags=["gateway"], metadata={"parts": kinds}),
+        root,
         langfuse.generation(
-            tid, "fabrix.chat", model=model_id,
-            startTime=datetime.fromtimestamp(started, timezone.utc).isoformat(), endTime=langfuse.now_iso(),
+            tid, "fabrix.chat", model=model_id, parent=root,
+            startTime=t0, endTime=langfuse.now_iso(),
             input=body.get("messages"), output="".join(col.text),
             usage=langfuse.usage_of(u.get("prompt_tokens"), u.get("completion_tokens")),
             level="ERROR" if status >= 400 else "DEFAULT", statusMessage="" if status < 400 else f"upstream {status}",