Nonce and challenge store lifecycle

[codebestia]

Write apps/backend/docs/concepts-nonce-lifecycle.md for lib/nonce.ts: how challenge nonces are minted, stored, consumed and expired, for both the sign-in challenge and the separate device-link challenge.

Acceptance criteria:

• Documents creation, single-use consumption and TTL for both nonce kinds
• Explains why device-linking uses a separate nonce namespace from sign-in, so hammering one flow cannot lock out the other
• Documents where nonces live and the behaviour across a restart or in a multi-node deployment
• Explains the replay resistance this provides and its limits

[Davichi-1]

@Davichi-1 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer! I’m interested in contributing to this task and can ensure timely delivery with clean implementation

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[BigManly4]

@BigManly4 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I'd like to contribute to this issue. As a full-stack and backend developer with open-source experience, I'll provide a high-quality implementation and keep the discussion updated throughout the process.

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[drips-wave]

Congratulations, @BigManly4! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

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[github-actions]

Closed by #599, merged into dev.

[drips-wave]

This issue has been completed by @BigManly4 as part of the Stellar Wave Program's 8th Wave 🥳

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