refactor: 整理文件夹结构，修复 create_serde 导入问题

- 移动 main_graph/tools/ 到 deprecated/main_graph_tools/（旧架构工具）
- 移动 rag_initializer.py 和 retry_utils.py 到 core/
- 清理 main_graph/nodes/ 里的旧节点到 deprecated/
- 修复 backend.py 中 create_serde 导入问题

backend/app/core/rag_initializer.py

@@ -0,0 +1,73 @@
+# app/rag_initializer.py
+from ...rag.tools import create_rag_tool
+from ...rag.retriever import create_parent_hybrid_retriever
+from ...model_services import get_embedding_service
+from backend.app.logger import info, warning
+import sys
+
+# 全局 RAG 工具
+_rag_tool = None
+_initialized = False
+
+
+def get_rag_tool() -> callable:
+    """获取全局 RAG 工具"""
+    return _rag_tool
+
+
+def is_initialized() -> bool:
+    """检查是否已初始化"""
+    return _initialized
+
+
+async def init_rag_tool(force: bool = False):
+    """
+    初始化 RAG 工具（注册到模块级变量，内部获取所需服务）
+
+    Args:
+        force: 是否强制重新初始化
+
+    Returns:
+        RAG 工具（@tool 装饰函数）或 None
+    """
+    global _rag_tool, _initialized
+
+    # 防止重复初始化
+    if _initialized and not force:
+        info("[RAG] 已初始化，跳过")
+        return _rag_tool
+
+    try:
+        from backend.app.model_services.chat_services import get_chat_service
+
+        info("🔄 正在初始化 RAG 检索系统...")
+        embeddings = get_embedding_service()
+        retriever = create_parent_hybrid_retriever(
+            collection_name="rag_documents",
+            search_k=5,
+            embeddings=embeddings,
+        )
+        rewrite_llm = get_chat_service()
+
+        rag_tool = create_rag_tool(
+            retriever=retriever,
+            llm=rewrite_llm,
+            num_queries=3,
+            rerank_top_n=5,
+        )
+
+        _rag_tool = rag_tool
+        _initialized = True
+        info(f"✅ RAG 检索工具初始化成功 (id={id(rag_tool)})")
+        return rag_tool
+
+    except Exception as e:
+        warning(f"⚠️ RAG 检索工具初始化失败: {e}")
+        return None
+
+
+def reset():
+    """重置（用于测试）"""
+    global _rag_tool, _initialized
+    _rag_tool = None
+    _initialized = False