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/flow/engine.rs

@@ -1,15 +1,40 @@
+// std
+use std::cell::RefCell;
 use std::collections::HashMap;
-use tokio::sync::{RwLock, Mutex};
-use futures::future::join_all;
-
-use rhai::Engine;
-use tracing::info;
 use std::time::Instant;
 
-// === 表达式评估支持：thread_local 引擎与 AST 缓存，避免全局 Sync/Send 限制 ===
-use std::cell::RefCell;
-use rhai::AST;
+// third-party
+use futures::future::join_all;
 use regex::Regex;
+use rhai::{AST, Engine};
+use tokio::sync::{Mutex, RwLock};
+use tracing::info;
+
+// crate
+use crate::flow::executors::condition::eval_condition_json;
+
+// super
+use super::{
+    context::{DriveOptions, ExecutionMode},
+    domain::{ChainDef, NodeKind},
+    task::TaskRegistry,
+};
+
+// 结构体：紧随 use
+pub struct FlowEngine {
+    pub tasks: TaskRegistry,
+}
+
+
+#[derive(Debug, Clone)]
+pub struct DriveError {
+    pub node_id: String,
+    pub ctx: serde_json::Value,
+    pub logs: Vec<String>,
+    pub message: String,
+}
+
+// === 表达式评估支持：thread_local 引擎与 AST 缓存，避免全局 Sync/Send 限制 ===
 
 // 模块：流程执行引擎（engine.rs）
 // 作用：驱动 ChainDef 流程图，支持：
@@ -132,12 +157,6 @@ pub(crate) fn eval_rhai_expr_json(expr: &str, ctx: &serde_json::Value) -> Option
         Err(_) => None,
     }
 }
-use super::{context::{DriveOptions, ExecutionMode}, domain::{ChainDef, NodeKind}, task::TaskRegistry};
-use crate::flow::executors::condition::eval_condition_json;
-
-pub struct FlowEngine {
-    pub tasks: TaskRegistry,
-}
 
 impl FlowEngine {
     pub fn new(tasks: TaskRegistry) -> Self { Self { tasks } }
@@ -491,14 +510,6 @@ impl Default for FlowEngine {
 }
 
 
-#[derive(Debug, Clone)]
-pub struct DriveError {
-    pub node_id: String,
-    pub ctx: serde_json::Value,
-    pub logs: Vec<String>,
-    pub message: String,
-}
-
 impl std::fmt::Display for DriveError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{}", self.message)