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/neo4j_manager.py

@@ -111,7 +111,7 @@ class Neo4jManager:
             # 验证连接
             self._driver.verify_connectivity()
             logger.info(f"Connected to Neo4j at {self.uri}")
-        except Exception as e:
+        except (RuntimeError, ValueError, TypeError) as e:
             logger.error(f"Failed to connect to Neo4j: {e}")
             self._driver = None