Easiest? Probably heroku after you get past the initial boilerplate - but can quickly become expensive.
Most cost efficient would be to Spin up a cheap droplet in digital ocean, install docker-compose with a Postgres service and python image. In the python image, just boot up a Django server on port 80. If you need https, youll need to add something in front of Django, like nginx to handle ssl termination.
App engine could also be a very quick and cheap way - you get ssl, and domain for free, but you do need to buy into their way of doing things, which has a bit of a learning curve .
Can’t really go wrong with python + Django (or flask if you want to stay minimal). Sounds like you’ll need auth, so Django should support that out of the box.
Both will integrate easily with Postgres, and for flask, take a look at the peewee ORM.