title: How to get notified each time a user's Auto-renewable subscription renews
body: <p>I am working on an app that has an auto-renewable subscription that gives users 200 tokens to use in the app each month. I have written a firebase cloud function to verify the receipt with apple and give the user the first 200 tokens on initial purchase, but I don't know how to continue to give the users a fixed amount of 200 tokens each month if their subscription is still valid. Could I store the receipt from the initial purchase of the subscription on my servers and use a monthly recurring firebase cloud function that checks the user's receipt from their initial purchase of the subscription to see if it's valid? Do old receipts tell you if a subscription is still valid like that? I also looked into Server to Server notifications but I am not very sure if the automatic renewal of a subscription gets sent as an event.</p>