fix(order): make escrow creation idempotent

[Adesokan-2]

Summary

• Reserve an order creation idempotency key before escrow deployment and bind it to a hash of the submitted request.
• Reuse a saved deployment and stable order ID on retries, so persistence failures do not deploy duplicate escrows.
• Add a buyer/seller scoped creation-status endpoint and regression coverage for post-deployment database failures and concurrent retries.

Closes #68

Checks

• git diff --check passed.
• Jest tests were not run because dependencies are not installed in this checkout.

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[Adesokan-2]

@Heazzy500 The idempotent order creation and recovery path for #68 is ready for review. Please take a look and merge when it is ready.