fix(payout): commit payout row and rotation index advance atomically

[JolaKay2000]

Overview

This PR makes the payout record and the rotation index advance a single atomic write in executePayout, so a recorded payment and a consumed rotation position can no longer disagree. The cycle-completion decision now follows from the committed state, and a failure between the two writes leaves neither applied — preventing a rotation position from stalling on a payment that was already made.

Related Issue

Changes

🔒 Atomic Payout Recording

• [MODIFY] src/services/payout.service.ts

  • Wrap the Payout row creation and the currentPayoutIndex advance in a single prisma.$transaction, so the recorded position and the consumed position commit together.
  • Move the cycle-completion decision to read from the committed transaction result rather than from state written outside it.
  • Keep the idempotency guard behavior intact: a payout that was committed is returned as-is, and the index is guaranteed to have advanced with it.

• [MODIFY] src/__tests__/payout.service.test.ts

  • Add a test simulating a failure between the payout row write and the index advance, asserting that neither is persisted and the rotation index does not stall.
  • Add a test asserting the payout row and index advance are committed together on success.

Verification Results

npm test -- src/__tests__/payout.service.test.ts
✅ payout.service tests passed (including new atomicity cases)

Acceptance Criteria	Status
The payout row and the rotation index advance are committed together	✅ Both writes run inside one prisma.$transaction
A failure at any point leaves either both or neither, with the payment recorded when it was made	✅ Transaction rollback covers the between-writes failure case
A rotation cannot stall on a position whose payment was already made	✅ Index advance is part of the same commit as the payout row
A test covers a failure between the two writes	✅ Added failure-injection test in payout.service.test.ts
npm test passes locally	✅ Verified locally

Closes #265

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