A guest coming back through their own already-redeemed link is the
ordinary #1265 case, and the identity the device holds is theirs. The same
link opened on a shared device that holds another guest's identity is not:
the redemption 409s, ensureIdentity() falls through to the stored identity,
and the visitor's likes are filed under the previous person.
The two cases were indistinguishable client-side, so the 409/410 body now
carries the invite's guest_id. On a mismatch the stored identity is cleared
and the visitor is asked who they are. A response without guest_id keeps
the previous behaviour.
Paul Nothaft
f2f40893c1