I recovered it just fine with a quick&dirty:

$ sqlite3 /var/guix/db.sqlite

delete from Refs where reference in (select id from ValidPaths where path glob "*libreof*");
delete from Refs where referrer in (select id from ValidPaths where path glob "*libreof*");
delete from DerivationOutputs where path glob "*libreof*";
delete from ValidPaths where path glob "*libreof*";

Which I assume is what `guix gc --verify=repair` was trying to do, but it's not cleaning up Refs table and thus failing at the FOREIGN KEY constraint.

$ sqlite3 /var/guix/db/db.sqlite

[...]

sqlite> .schema

[...]

CREATE TABLE Refs (
    referrer  integer not null,
    reference integer not null,
    primary key (referrer, reference),
    foreign key (referrer) references ValidPaths(id) on delete cascade,
    foreign key (reference) references ValidPaths(id) on delete restrict <-------- this one here
);