fix: auto-fix code issues (cron)

- 修复重复导入/字段
- 修复异常处理 (将裸 except Exception 改为具体异常类型)
- 修复PEP8格式问题
- 清理未使用导入
- 添加 UUID_LENGTH 常量替代魔法数字
- 添加 DEFAULT_RATE_LIMIT, MASTER_KEY_RATE_LIMIT, IP_RATE_LIMIT 常量
- 添加 MAX_TEXT_LENGTH, DEFAULT_TIMEOUT 常量

涉及文件:
- backend/main.py
- backend/db_manager.py
- backend/llm_client.py
- backend/neo4j_manager.py
- backend/tingwu_client.py
- backend/tenant_manager.py
- backend/growth_manager.py
- backend/workflow_manager.py
- backend/image_processor.py
- backend/multimodal_entity_linker.py
- backend/multimodal_processor.py
- backend/plugin_manager.py
- backend/rate_limiter.py

backend/tingwu_client.py

@@ -8,6 +8,7 @@ import time
 from datetime import datetime
 from typing import Any
 
+
 class TingwuClient:
     def __init__(self):
         self.access_key = os.getenv("ALI_ACCESS_KEY", "")
@@ -67,7 +68,7 @@ class TingwuClient:
             # Fallback: 使用 mock
             print("Tingwu SDK not available, using mock")
             return f"mock_task_{int(time.time())}"
-        except Exception as e:
+        except (RuntimeError, ValueError, TypeError) as e:
             print(f"Tingwu API error: {e}")
             return f"mock_task_{int(time.time())}"
 
@@ -107,7 +108,7 @@ class TingwuClient:
         except ImportError:
             print("Tingwu SDK not available, using mock result")
             return self._mock_result()
-        except Exception as e:
+        except (RuntimeError, ValueError, TypeError) as e:
             print(f"Get result error: {e}")
             return self._mock_result()