I did hunt around a while back to find this as a SaaS application. Couldn't find it. With an authorizer, you could launch!
Under heavy workload on a specific counter, this is going to hotspot the item in DynamoDB. As it has to be atomic, there is no way to cache (check you haven't got any caching in API Gateway) and the usual Scatter-Gather pattern defeats the atomic nature too.
Fun though.