[Backend] JWT authentication middleware

[libby-coder]

Wallet-signature-based auth: frontend signs a nonce with Freighter, backend verifies signature and issues a short-lived JWT for write operations.

[PrevailUgah]

@PrevailUgah has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer! I'd love to work on this issue, please assign it to me.

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

[Okorie2000-code]

@Okorie2000-code has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer,

I'd love to work on this issue. I can implement a secure wallet-signature authentication flow with robust JWT handling, ensuring the solution is reliable, maintainable, and aligned with the project's security requirements.

Thank you for your consideration.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Okorie2000-code to this issue.

[drips-wave]

Congratulations, @PrevailUgah! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @PrevailUgah: 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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