A login stores the username in the session cookie, which cannot carry
a value past about 4096 bytes, so a username of about 2 KB or more
could never log in and the login answered 500.
Usernames are now limited to 1024 bytes, about half of what the cookie
can carry. The User model rejects a longer one with
ErrUsernameTooLong, and a check constraint on the users table rejects
it for any path that bypasses the model. The comment on
MaxUsernameBytes gives the arithmetic.
Model: opus-5-5
clawbot