[feature] Google Play Billing adapter for the verified badge

[aquie00t]

The store adapter behind the badge. Depends on the provider-agnostic core.

Digital goods sold inside an Android app must go through Play Billing - a card form in the app is not an option. Google becomes the merchant of record, which also removes the Turkish consumer-invoicing problem a web payment gateway would have created.

Tasks

• Play Console: subscription product and base plan at 100 TL / month.
• Service account + Play Developer API for server-side purchase verification. The client's word is never enough.
• Real-time developer notifications (Pub/Sub) into the existing webhook shape - renewals, cancellations, refunds, grace and hold states.
• Server-side cancellation on ban and on account deletion. Play supports this, so the promise that a banned or deleted account stops being charged can actually be kept here.
• Apply for the 15% small-business commission tier; it is not automatic.

Note for later: the same promise cannot be kept on iOS. Apple gives developers no way to cancel a subscription or refund from the server - see the App Store items in this project.