Server now holds the secret in an atomic.Pointer so a running instance can
swap it; outstanding challenge and verification tokens signed with the old
secret immediately fail verification.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
VerifyToken now consumes a redemption token on its first successful verify:
a replayed token within its 5-min TTL is rejected. Mirrors the existing
single-use challenge-nonce store with a per-Server used-token store keyed on
the token's random id, same GC/expiry approach (entries live only for the
remaining TTL). The token is burned only on a successful verify (valid HMAC +
unexpired + unused); forged/expired tokens never touch the store.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
