feat: add dag_engine app — CLI + web frontend for DAG execution (0007e)

Full DAG engine app with CLI subcommands (run, list, status, validate, server)
and React/Mantine web frontend. Uses net/http + embedded Vite build. SQLite
store for run history. Scheduler with cron_ticker for automated execution.
Compatible with existing dagu YAML format.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/dag_engine/handlers_scheduler.go

@@ -0,0 +1,27 @@
+package main
+
+import "net/http"
+
+func handleSchedulerStart(scheduler *Scheduler) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		if err := scheduler.Start(); err != nil {
+			writeError(w, http.StatusConflict, err.Error())
+			return
+		}
+		writeJSON(w, http.StatusOK, map[string]string{"status": "started"})
+	}
+}
+
+func handleSchedulerStop(scheduler *Scheduler) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		scheduler.Stop()
+		writeJSON(w, http.StatusOK, map[string]string{"status": "stopped"})
+	}
+}
+
+func handleSchedulerStatus(scheduler *Scheduler) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		status := scheduler.Status()
+		writeJSON(w, http.StatusOK, status)
+	}
+}