Bound username length at creation (closes #184)

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

internal/handlers/logbound_test.go

@@ -339,11 +339,9 @@ const storedUserPassword = "correct-horse-battery-staple"
 // storedFillBytes is the raw length of the client-chosen value in
 // those accounts' usernames. It is well past the 512-byte field
 // budget, so the line is still truncated, but short enough that the
-// session cookie a successful login writes stays inside
-// securecookie's 4 KB limit: the cookie is written BEFORE the
-// "user logged in" line, so an 8 KB username answers 500 and never
-// reaches it.
-const storedFillBytes = 1024
+// whole username, markers and fill name included, stays within
+// database.MaxUsernameBytes.
+const storedFillBytes = 960
 
 // storedFill builds a username fill of storedFillBytes raw bytes out
 // of repetitions of ch, with both markers at its far end.