frontend: sync model with migrations
    
    Some indexes/constraints were added without modifying the model, and
    some were added without following the normal alembic naming pattern.
    
    The newly model-defined key reordered the list of BuildChroots in notify
    testsuite (the testsuite now better matches production reality) so I
    edited the testsuite to use sets, not lists.
    
    Fixes: #1346