cleanUsername is many-to-one, so distinct usernames (e.g. "teamone/x" and
"teamone-x") can normalize to the same home directory. With CreateUserDir
enabled, the second registrant silently reused the first user's directory,
breaking per-user isolation. Add a GetByScope lookup and reject a signup whose
derived scope is already taken. The check is gated on CreateUserDir: when it is
off, signups intentionally share the configured default scope. Adds a regression
test for the colliding-username case.
