feat(scheduler): 定时任务后端持久化 + Pipeline trigger 编译修复

S4/S8 定时任务后端:
- 新增 scheduled_tasks 表 (migration v7)
- 新增 scheduled_task CRUD 模块 (handlers/service/types)
- 注册 /api/scheduler/tasks 路由 (GET/POST/PATCH/DELETE)
- 新增 start_user_task_scheduler() 30秒轮询循环
- 支持 cron/interval/once 三种调度类型
- once 类型执行后自动禁用

修复:
- pipeline_commands.rs: 修复 pipeline.trigger 字段不存在的编译错误
  (Pipeline 结构体无 trigger 字段，改用 metadata.tags/description)

desktop/src-tauri/src/pipeline_commands.rs

@@ -1081,13 +1081,12 @@ pub async fn route_intent(
     let mut parser = TriggerParser::new();
 
     for (id, pipeline) in pipelines.iter() {
-        // Extract trigger info from pipeline metadata
-        // For now, use tags as keywords and description as trigger description
+        // Derive trigger info from pipeline metadata (tags as keywords, description)
         let trigger = Trigger {
             keywords: pipeline.metadata.tags.clone(),
-            patterns: vec![],  // TODO: add pattern support in pipeline definition
+            patterns: vec![], // Patterns not defined in Pipeline struct
             description: pipeline.metadata.description.clone(),
-            examples: vec![],
+            examples: vec![], // Examples not defined in Pipeline struct
         };
 
         // Convert pipeline inputs to trigger params