Detecting paid app plan purchase / change from middleware

We are building a paid app for zendesk marketplace which involves a middleware. We have a requirement to know whenever the customer purchases / changes the app plan.

To give more context, our middleware performs data sync between zendesk and an external system. Based on the plan that the customer has subscribed, we would want to change the sync behaviour & few other functionalities.
We found that the zaf client provides the stripe subscription ID & the app plan in the frontend metadata but we want it in our middleware and also a way to detect the change.