Security: GPU Escrow Concurrency Fix

[MichaelSovereign]

Summary

This PR fixes a critical race condition in the GPU escrow creation logic that could allow users to lock more funds than they actually possess.

Changes

1. Atomic Escrow Initiation: Wrapped the entire escrow creation process (balance check, balance deduction, and escrow record insertion) within a SQLite transaction. This ensures that concurrent requests from the same wallet are processed serially, preventing 'over-escrowing' where the balance check passes for multiple requests before the first deduction is committed.
2. Robust Rollbacks: Added explicit rollbacks on balance check failures to ensure database integrity.

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.)