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.