The supersede DELETE in CreatePasskeyChallenge filtered consumed_at IS NULL, so it only reaped the prior LIVE challenge; the row that each finish stamps consumed_at on was left behind. A begin->finish loop therefore accumulated one dead row per cycle, unbounded. Drop the consumed_at clause so a fresh begin reaps ALL prior rows for (user, purpose), bounding the table at one row per (user, purpose) with zero net growth per cycle. Deleting an already-consumed row is safe: it has been redeemed and nothing reads it. The fake mirrors the widened supersede.
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