The “postgresql-server” package does not ship with a PAM service file (in /etc/pam.d/). The official PostgreSQL documentation states that the default PAM service name is “postgresql”, so by default PostgreSQL Server looks for a PAM service file of this name (although it can be changed in pg_hba.conf I think). This makes PAM authentication with PostgreSQL Server broken out of the box I believe.
I couldn’t find any openSUSE documentation on this specifically. Is it the intention for all server admins to write their own PAM service file? Is this a bug?