The shared server-nonce store every signed action consumes (admin login,
first-run setup, self-service erasure). DB-backed so consumption is atomic and
correct under concurrent requests and a future load-balanced fleet:
DELETE FROM challenges WHERE nonce=$1 AND purpose=$2 AND expires_at > NOW()
RETURNING subject
Winning the race returns one row; zero rows = replay/expired/wrong-purpose =>
reject (no TOCTOU). issue_challenge mints a 256-bit nonce bound to a purpose and
optional subject; consume_challenge is the atomic single-use step that pairs with
verify_signed_action (already built, §0.1). Periodic expiry sweep bounds the
table; the table holds no secret and is documented as backup-excluded
(ephemeral, links pubkey->time).
migration 20260625000001; ConsumedChallenge exported; 3 L1 tests (consume-once /
replay-rejected, wrong-purpose rejected, expired rejected).
