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/tenant_manager.py

@@ -395,7 +395,7 @@ class TenantManager:
             conn.commit()
             logger.info("Tenant tables initialized successfully")
 
-        except Exception as e:
+        except (RuntimeError, ValueError, TypeError) as e:
             logger.error(f"Error initializing tenant tables: {e}")
             raise
         finally:
@@ -760,7 +760,7 @@ class TenantManager:
 
             return is_verified
 
-        except Exception as e:
+        except (RuntimeError, ValueError, TypeError) as e:
             logger.error(f"Error verifying domain: {e}")
             return False
         finally: