REVIEW-001 (M0) L5s: a code valid for ±90s could be replayed. TOTP is a
login fallback (D-005) and the only factor the bootstrap admin has (D-041),
so a code observed once (shoulder-surfed, phished, in a log) must not work
twice.
Track the highest accepted counter per identity and refuse any counter that
is not strictly newer. The search is newest-first and compares in constant
time; a replay is reported as ErrInvalidCode — indistinguishable from a wrong
code, so the response is not an oracle (A3). A fresh secret (re-enrollment)
resets the counter.