Willy-nilly fixes

[ineiti]

Just to put this out there - with these fixes, mostly created by Claude Code, and in no way peer-reviewed other than "manually testing seems to make this work", I was able to:

• create a conference with two tracks
• correctly attribute rooms to it
• set up questions to be asked during signup
• send bulk mails to attendees on different criteria
• edit users: adding / removing / modifying

It's very raw, and probably should not be merged in this form. But if somebody wants to use osem, it's definitely better than the 'master' branch.

Closes #3688