feat(ws): 新增WebSocket实时通信支持与SSE独立服务

重构中间件结构，新增ws模块实现WebSocket流程执行实时推送
将SSE服务拆分为独立端口监听，默认8866
优化前端流式模式切换，支持WS/SSE协议选择
统一流式事件处理逻辑，完善错误处理与取消机制
更新Cargo.toml依赖，添加WebSocket相关库
调整代码组织结构，规范导入分组与注释

backend/src/services/flow_service.rs

@@ -1,5 +1,3 @@
-// removed unused: use std::collections::HashMap;
-// removed unused: use std::sync::Mutex;
 use anyhow::Context as _;
 use serde::{Deserialize, Serialize};
 
@@ -8,8 +6,6 @@ use crate::flow::{self, dsl::FlowDSL, engine::FlowEngine, context::{DriveOptions
 use crate::db::Db;
 use crate::models::flow as db_flow;
 use crate::models::request_log; // 新增：查询最近修改人
-use crate::services::flow_run_log_service;
-use crate::services::flow_run_log_service::CreateRunLogInput;
 use sea_orm::{EntityTrait, ActiveModelTrait, Set, DbErr, ColumnTrait, QueryFilter, PaginatorTrait, QueryOrder};
 use sea_orm::entity::prelude::DateTimeWithTimeZone; // 新增：时间类型
 use chrono::{Utc, FixedOffset};