feat(flow): 重构流程引擎与任务执行器架构

重构流程引擎核心组件，引入执行器接口Executor替代原有TaskComponent，优化节点配置映射逻辑：
1. 新增mappers模块集中处理节点配置提取
2. 为存储层添加Storage trait抽象
3. 移除对ctx魔法字段的依赖，直接传递节点信息
4. 增加构建器模式支持引擎创建
5. 完善DSL解析的输入校验

同时标记部分未使用代码为allow(dead_code)

backend/src/services/flow_service.rs

@@ -246,7 +246,7 @@ pub async fn run(db: &Db, id: &str, req: RunReq, operator: Option<(i64, String)>
          };
          let mut ctx = req.input.clone();
          // Merge node-scoped configs into ctx under ctx.nodes
-         let supplement = flow::dsl::ctx_from_design_json(design);
+         let supplement = flow::mappers::ctx_from_design_json(design);
          merge_json(&mut ctx, &supplement);
          // 解析 executionMode / execution_mode
          let mode_str = design.get("executionMode").and_then(|v| v.as_str())
@@ -335,7 +335,7 @@ pub async fn run(db: &Db, id: &str, req: RunReq, operator: Option<(i64, String)>
 
     // 从全局注册中心获取任务（若未初始化则返回默认注册表）
     let tasks: flow::task::TaskRegistry = flow::task::get_registry();
-    let engine = FlowEngine::new(tasks);
+    let engine = FlowEngine::builder().tasks(tasks).build();
 
     info!(target = "udmin", "flow.run: driving engine id={} nodes={} links={} execution_mode={:?}", id, chain.nodes.len(), chain.links.len(), exec_mode);
     // 执行