Add idempotency for payment operations to prevent double charges

[Smartdevs17]

Description

Network retries can cause duplicate charges. Implement idempotency keys for all payment operations.

Acceptance Criteria

• Idempotency-Key header support
• Key deduplication window (24h)
• Response caching for idempotent requests
• Clear error on key collision
• Key lifecycle management with cleanup
• Client documentation for idempotency

Technical Scope

• backend/services/
• Edge: key reuse on failed operations, storage limits for key cache

[alexatsejames-alt]

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

Hi can you please assign these issue to me

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

[drips-wave]

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

🧑‍💻 @alexatsejames-alt: 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

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