feat(dag_engine): WS hub /api/ws/dagruns + migracion DAGs desde dagu

- events.go: DagRunHub broadcastea snapshot+deltas live (500ms tick, 5s recent finished window) sobre dag_runs + dag_step_results.
- api.go: handler GET /api/ws/dagruns upgrade WS, opt-in en RegisterAPI.
- store.go: expone Conn() para read-only desde el hub.
- main.go: construye DagRunHub al arrancar server.
- dags_migrated/: 5 YAMLs migrados desde ~/dagu/dags tras desinstalar dagu (issue 0095 step 1).

Smoke: snapshot inicial OK, trigger /api/dags/test_claude_access/run -> delta WS observa 3 step_results + run success en <1s.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/dag_engine/main.go

@@ -286,6 +286,7 @@ func cmdServer(args []string) {
 
 	executor := NewExecutor(db, cfg.DagsDir)
 	scheduler := NewScheduler(executor, cfg.DagsDir)
+	dagRunHub := NewDagRunHub(db, executor)
 
 	// Prepare frontend FS.
 	var feFS iofs.FS
@@ -303,7 +304,7 @@ func cmdServer(args []string) {
 	}
 
 	mux := http.NewServeMux()
-	RegisterAPI(mux, executor, scheduler, feFS)
+	RegisterAPI(mux, executor, scheduler, dagRunHub, feFS)
 
 	handler := corsMiddleware(loggingMiddleware(mux))