There are plenty of proxies that do that with some config like nginx. Even if you were using a relational database with a backend you’d still have to solve the same problem.
One of the major selling point of couchdb is replication protocol for client-server data syncing. When you design product with posgress you don't allow to execute raw sql queries from clients without any application server. But looks like it is recommended way to update data in couchdb world if you want to have synchronisation. I can't understand how can this architecture be secure?