Implement vseth login

lgin was the easy part. the hard part is, that the vseth people id is a 32byte uuid. while the amiv id is a 24byte str. luckily the field is already 32bytes. but there needs to be a good way to differentiate between the two login systems, in a way where there isn't to much code overhead. the system now converts all amiv IDs to uuids at the login. after that everything is a uuid. the db migration is a bit more complicated due to that but it should be possible.

also the invoice generating system has been rewriten in typst.