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/migrations/migrate_orphaned_documents.py

@@ -1,12 +1,15 @@
 """
 迁移孤立文档到默认知识库
 """
+import logging
 from sqlalchemy.orm import Session
 from ..core.database import get_db
 from ..models.user import User
 from ..models.knowledge_base import KnowledgeBase
 from ..models.document import Document
 
+logger = logging.getLogger(__name__)
+
 
 def migrate_orphaned_documents():
     """将没有知识库的文档迁移到用户的默认知识库"""
@@ -40,12 +43,12 @@ def migrate_orphaned_documents():
                     doc.knowledge_base_id = default_kb.id
                 
                 db.commit()
-                print(f"用户 {user.username} 的 {len(orphaned_docs)} 个文档已迁移到默认知识库")
+                logger.info(f"用户 {user.username} 的 {len(orphaned_docs)} 个文档已迁移到默认知识库")
         
-        print("孤立文档迁移完成")
+        logger.info("孤立文档迁移完成")
         
     except Exception as e:
-        print(f"迁移失败: {str(e)}")
+        logger.error(f"迁移失败: {str(e)}")
         db.rollback()
     finally:
         db.close()