fix: auto-fix code issues (cron)

- 修复隐式 Optional 类型注解 (RUF013)
- 修复不必要的赋值后返回 (RET504)
- 优化列表推导式 (PERF401)
- 修复未使用的参数 (ARG002)
- 清理重复导入
- 优化异常处理

backend/document_processor.py

@@ -78,7 +78,7 @@ class DocumentProcessor:
                     "PDF processing requires PyPDF2 or pdfplumber. Install with: pip install PyPDF2",
                 )
         except Exception as e:
-            raise ValueError(f"PDF extraction failed: {str(e)}")
+            raise ValueError(f"PDF extraction failed: {e!s}")
 
     def _extract_docx(self, content: bytes) -> str:
         """提取 DOCX 文本"""
@@ -109,7 +109,7 @@ class DocumentProcessor:
                 "DOCX processing requires python-docx. Install with: pip install python-docx",
             )
         except Exception as e:
-            raise ValueError(f"DOCX extraction failed: {str(e)}")
+            raise ValueError(f"DOCX extraction failed: {e!s}")
 
     def _extract_txt(self, content: bytes) -> str:
         """提取纯文本"""