[BACKEND] Add an idempotency key to the transfer endpoint

[Trovic1]

Summary

Accept an optional Idempotency-Key header so a retried transfer request does not double-charge.

Scope

• backend only

Likely files

• src/routes/WalletRoutes.js
• src/services/WalletService.js
• src/models/Transaction.js

Acceptance criteria

• Same key + same payload returns the original result without re-running the transfer
• Keys expire or are scoped per user
• Works within the existing atomic transaction flow

Verification

• Replay an identical transfer with the same key and confirm only one debit

Labels: backend, enhancement, help wanted

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