Account: a session signed in with a row's old key keeps drive access after a link #2864

Follow-up from #2843 (account linking).

A link moves a row from its old relay key to the account key and deliberately leaves the row's sessions open. Account actions (approving, introducing, enrolling, starting a code through the relay) already require the session's proven key to be the row's current key. A session that signed in with the old key keeps ordinary drive access (terminals, settings) until it reconnects.

Expected: decide whether a link should end other sessions that signed in with the old key (close them with auth_changed once the link is written, keeping the linking session's response), and cover it with a contract test.