Fix critical sales and backup regressions

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Summary

• Stop sale detail GET requests from rewriting persisted sale totals.
• Reject unsupported sale payment methods and remove the exposed credit checkout option.
• Make draft sale item deletion POST-only with CSRF and transactional row locks.
• Restore backup restore/delete UI compatibility and nullable admin log entries for log file actions.

Validation

• mkdir -p logs && .venv/bin/python manage.py test inventory.tests.test_sale_status inventory.tests.test_sales_balance_payment inventory.tests.test_views.BackupViewSecurityTest inventory.tests.test_views.LogFileViewTest
• mkdir -p logs && .venv/bin/python manage.py check