Add postgres setup to playbook

[jeriox]

but lets not write that role for ourselves. Jim has something nice prepared (https://git.2martens.de/2martens/uberspace-ansible/src/branch/master/roles/postgres), although it seems a bit outdated because this role compiles postgres manually. https://githubmemory.com/repo/friesenkiwi/ansible-collection-uberspace also announced a role based on the UberLab guide

Originally posted by @jeriox in #6 (comment)