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/store/store.go

@@ -36,6 +36,12 @@ func (db *DB) Close() error {
 	return db.conn.Close()
 }
 
+// Conn exposes the underlying *sql.DB for read-only queries from other
+// packages (e.g. WS hub in events.go). Do not Close() the returned conn.
+func (db *DB) Conn() *sql.DB {
+	return db.conn
+}
+
 // --- DagRun CRUD ---
 
 // DagRun mirrors infra.DagRun for the store layer.