-- ID secuencial humano por card. Distinto del id hex (PK interna).
-- Backfill por orden de creacion.
ALTER TABLE cards ADD COLUMN seq_num INTEGER NOT NULL DEFAULT 0;
UPDATE cards SET seq_num = (
    SELECT COUNT(*) FROM cards c2 WHERE c2.created_at <= cards.created_at
) WHERE seq_num = 0;
CREATE UNIQUE INDEX IF NOT EXISTS idx_cards_seq_num ON cards(seq_num) WHERE seq_num > 0;