Is 'app/uninstalled' emitted only when the app which created the webhook is removed? Or any app? If it's any app, how can I know that MY app was removed so I can delete stuff in my database?
you must create a webhook('app/uninstalled') during the installation flow.
The webhook create will come triggered when an user perform installation, don't forget to setup a link for webhook who will receive the data for unistallation.
I hope I have been helpful.