If you can scale your app to zero, couldn’t you also just scale your database to zero once no more app servers are running? Or for try-out apps, as you mention

If you can scale your app to zero, couldn’t you also just scale your database to zero once no more app servers are running? Or for try-out apps, as you mention, you could just run Postgres next to your app in the same container. This might be possible with fly.io, or will soon, I think. I’m not sure how comfortable I am using a custom flavor of Postgres (even if it’s just the storage layer).