Laddr doesn't like Underscores in usernames

We have noticed that Laddr doesn't like underscores in usernames.

Start by reproducing the reported problem with a username containing an underscore, then trace where usernames are validated or processed. The issue does not name a file or test, so locate the relevant entry point before deciding the change; done means such usernames are handled correctly and the behavior is covered by a regression check.

索引モデルが issue の本文から書いたものです。