fix(consensus): use atomic debit/credit in execute_transfer

[Sertug17]

Fixes #1734

Summary

execute_transfer used a non-atomic read→check→write pattern for balance updates. Since _process_finalization_task creates a separate DB session per task, two concurrent finalizations crediting the same recipient could both read the same stale balance the second write silently overwrites the first, losing funds with no error.

AccountsManager already had atomic SQL alternatives that were used elsewhere but not here.

Changes

• backend/consensus/base.py switched sender debit to debit_account_balance (SQL UPDATE WHERE balance >= amount) and recipient credit to credit_account_balance (SQL UPDATE balance + amount). UNDETERMINED short-circuit path preserved.

Test plan

• pytest tests/db-sqlalchemy/test_execute_transfer_undetermined_finalization.py passes
• Integration tests pass

[github-actions]

This PR targeted main, which is only the default/static branch.

I retargeted it to v0.123-dev, the active development branch. Pushes to v0.123-dev automatically fast-forward main.

[coderabbitai]

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