fix(oauth): require account evidence before migrating legacy GitHub bindings

Legacy GitHub bindings stored the login name, while current bindings store
the numeric account ID. A legacy match no longer signs the user in on its
own. When the account has two-factor or a passkey, the binding is rewritten
only after that verification completes, inside the transaction that issues
the session. Otherwise it is rewritten when one of the GitHub account's
verified emails matches the account email. Without either, the login is
declined with guidance to sign in another way and relink GitHub in account
settings. All-digit login names never take part in the comparison, the bind
flow no longer compares login names, and every migration or declined
attempt is recorded as an account binding audit event.