Apps return 502 when opened immediately after start or restart

After starting or restarting an app, Umbrel can mark it as ready before the app's internal HTTP upstream has started accepting connections.

If the app is opened immediately, the first request may fail with an ECONNREFUSED / HTTP 502 error. Waiting briefly and opening the app again usually works.

Steps to reproduce

1. Start or restart an App Store app that uses app_proxy.
2. Wait for the loading indicator to disappear.
3. Open the app immediately.
4. Observe the 502 error page.
5. Wait a few seconds and open the app again.
6. The app now loads successfully.

Screen recording

https://github.com/user-attachments/assets/5e482b8e-4e51-459d-b78e-22e877b21e01