Adds displayname to existing assertions and new tests covering:
null/short/invalid-charset displayname, duplicate displayname,
findability by displayname. AuthenticateUser tests pick up the
seedUser displayname argument.
9 cases: null/empty/malformed email -> BadRequest; null or
sub-8-char password -> BadRequest; duplicate email -> DomainException;
valid signup returns User with hashed password and isAdmin=false;
user is findable by email afterwards; EmailAddress vo lowercases
the domain. fails red - SignupUser class not yet defined.
