[Backend] Replace unsafe wallet-secret flows with hardened secret management

[Solomonsolomonsolomon]

Eliminate plaintext or loosely handled secret transit/storage assumptions across signup, user persistence, and execution services. Introduce encrypted-at-rest handling, safer API boundaries, migration strategy, and tightly controlled decryption points.nn- Priority: Highn- Area: Auth / Security / Datan- Advanced Scope: secret lifecycle, migration, trust boundary redesign

[Emelie-Dev]

@Emelie-Dev has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi, I would love to work on this issue.

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[drips-wave]

Congratulations, @dev-chinasa! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @dev-chinasa: 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.

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