`/api/auth/sso/complete` only accepted cookie slots 0-4 and quietly
used slot 0 for anything higher. Since the account limit rose to
`MAX_ACCOUNT_SLOTS` (50) on HTTP/2, adding a sixth account through
server-side SSO wrote its refresh token over the first account's cookie,
while the new account was registered on slot 5 with no cookie behind it.
The route and the OAuth callback's pending-slot check now use the same
bound as every other auth route.
Linus Rath
4db88255e8