* fix(backend): reject a replayed TOTP code within its validity window
verifyTotp() was stateless — otplib's window:1 tolerance meant the
same 6-digit code could complete two independent logins inside its
~90s validity window. Track each admin's last-consumed step and
reject a code that doesn't advance past it.
* fix(backend): make the TOTP replay-tracking persist atomic
verifyTotpEncryptedStep() read two_factor_last_used_step, then a plain
UPDATE wrote the new step with no conditional guard — two concurrent
requests carrying the same captured code could both pass the check
before either UPDATE landed. The persist is now a conditional UPDATE
(only advances the step, checked via affected-row count), so a losing
concurrent request is correctly treated as a replay.
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Co-authored-by: Paul Nothaft <paul@MacStudio-von-Paul.local>
Paul Nothaft
cdde937d7f