refactor: replace print() with structured logging across backend

Replace all print()/stderr logging with Python logging module using
logger = logging.getLogger(__name__) pattern for consistent log levels
and formatting. Extract score conversion to shared score_utils module.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/rag/vector_store.py

@@ -2,6 +2,7 @@
 LangChain 1.0 向量存储封装
 """
 import os
+import logging
 import json
 import hashlib
 from typing import List, Dict, Any, Optional
@@ -12,6 +13,8 @@ from langchain_core.documents import Document as LangChainDocument
 from ..core.config import get_settings
 from .embeddings import get_embedding_model
 
+logger = logging.getLogger(__name__)
+
 settings = get_settings()
 
 
@@ -40,7 +43,7 @@ class VectorStore:
             self.vectorstore.add_documents(documents)
             return True
         except Exception as e:
-            print(f"添加文档失败: {str(e)}")
+            logger.error(f"添加文档失败: {str(e)}")
             return False
     
     def as_retriever(self, **kwargs):
@@ -54,7 +57,7 @@ class VectorStore:
         filter: Optional[Dict] = None
     ):
         """相似度搜索（带分数）"""
-        print(f"[DEBUG-VectorStore] 查询参数 - k: {k}, filter: {filter}")
+        logger.debug(f"查询参数 - k: {k}, filter: {filter}")
         
         result = self.vectorstore.similarity_search_with_score(
             query=query,
@@ -62,7 +65,7 @@ class VectorStore:
             filter=filter
         )
         
-        print(f"[DEBUG-VectorStore] 返回结果数量: {len(result)}")
+        logger.debug(f"返回结果数量: {len(result)}")
         return result
     
     def delete_by_document_id(self, document_id: int) -> bool:
@@ -73,7 +76,7 @@ class VectorStore:
             )
             return True
         except Exception as e:
-            print(f"删除向量数据失败: {str(e)}")
+            logger.error(f"删除向量数据失败: {str(e)}")
             return False
 
     def max_marginal_relevance_search(