refactor(architecture): consolidate the two passkey-pepper mechanisms into config

[cypriannwokolo2-creator]

Summary

auth_handler.go:734 uses h.security.WalletPepper (config) while getPasskeyPepper() (:760-766) reads env MOISTELLO_PASSKEY_PEPPER directly and is otherwise unused; config.SecurityConfig.PasskeyPepper is loaded but unused.

Acceptance Criteria

• Single source of truth for passkey pepper (config struct)
• Remove direct env reads in handlers
• Env/config wiring verified end-to-end
• Tests

Complexity

Medium-High.

[Alagba467]

@Alagba467 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer I would like to work on this issue, thank you.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Alagba467 to this issue.

[simplex001]

@simplex001 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hello maintainer, i'd love to work on this issue! please assign me. I have reviewed the issue details and fully understand the requirements

ℹ️ Repo Maintainers: To accept this application, review their application or assign @simplex001 to this issue.

[drips-wave]

Congratulations, @simplex001! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @simplex001: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊