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/document_loaders.py

@@ -1,6 +1,7 @@
 """
 LangChain 1.0 文档加载器封装 + PDF图片提取
 """
+import logging
 from typing import List, Optional, Dict
 from pathlib import Path
 import fitz  # pymupdf
@@ -12,6 +13,8 @@ from langchain_community.document_loaders import (
 )
 from langchain_core.documents import Document
 
+logger = logging.getLogger(__name__)
+
 
 class PDFImageExtractor:
     """使用pymupdf从PDF中提取内嵌图片"""
@@ -59,7 +62,7 @@ class PDFImageExtractor:
                         "size": len(image_bytes),
                     })
                 except Exception as e:
-                    print(f"[ImageExtractor] 提取图片失败 page={page_num+1} img={img_idx}: {e}")
+                    logger.warning(f"提取图片失败 page={page_num+1} img={img_idx}: {e}")
                     continue
 
         doc.close()