fix: make SimulateEngine.stop() synchronous

The stop() method was async but called from a sync context,
causing 'RuntimeError: no running event loop'. Changed to sync
since it just sets flags.

src/backend/app/api/simulate.py

@@ -226,7 +226,7 @@ def stop_simulation(
 
     if run_id in running_simulations:
         engine = running_simulations[run_id]
-        asyncio.create_task(engine.stop())
+        engine.stop()
         simulation.status = "stopped"
         db.commit()