Security: GPU Refund Integrity & Concurrency Fix

[MichaelSovereign]

Summary

This PR hardens the GPU escrow refund process by ensuring atomicity and implementing stricter authorization checks.

Changes

1. Atomic Refund Transaction: Wrapped the entire refund process (state check, state update, and balance restoration) in a transaction. This prevents race conditions where concurrent release and refund requests might both succeed, leading to double-spending or fund discrepancies.
2. Strict Authorization: Enforced that only the service provider () can authorize a refund. Previously, the check was less restrictive.
3. Data Integrity: Added explicit rollbacks on all error paths to maintain database consistency.

Closes #6460

[Scottcjn]

Closing as part of Tier 0 hard-ban cleanup — see #3074 / #3104 / #3169 for the documented incident chain. All MichaelSovereign PRs are closed unread per the Tier 0 contract. No review path; no future PRs from this account will be processed. (See feedback_michaelsovereign_tier0_2026-05-02.md.)