Add FastAPI backend with agent system

backend/app/config_tracing.py

@@ -0,0 +1,26 @@
+"""
+LangSmith Tracing 配置
+提供 Callback 工厂函数，用于 LangGraph 追踪
+"""
+
+from langchain_core.tracers import LangChainTracer
+
+from app.config import settings
+
+
+def get_langsmith_callbacks() -> list:
+    """
+    根据配置返回 LangSmith Callback 列表
+    未启用时返回空列表
+    """
+    if not settings.LANGSMITH_TRACING:
+        return []
+
+    if not settings.LANGSMITH_API_KEY:
+        return []
+
+    return [
+        LangChainTracer(
+            project_name=settings.LANGSMITH_PROJECT,
+        )
+    ]