The uniqueness check on workspace invitations only filtered by
status='pending', not by expires_at. Combined with the partial unique
index idx_invitation_unique_pending (also keyed only on status), a
past-due pending row permanently blocked re-inviting the same email.
Now, before creating a new invitation, the handler flips any past-due
pending row for the same (workspace_id, invitee_email) to 'expired',
freeing the unique slot. Also tightens GetPendingInvitationByEmail to
require expires_at > now(), matching the existing list queries.
Closesmultica-ai/multica#2055.
5c42ed1649