Fix critical sales and backup regressions

[cursor]

Summary

• Stop sale detail GET requests from rewriting persisted sale amounts.
• Reject unsupported sale payment methods and remove exposed credit checkout.
• Require POST/CSRF and transaction locks for draft sale item deletion.
• Fix backup restore confirmation/context and roll back DB/media when media restore fails.
• Avoid invalid admin log content type IDs for log file actions.

Validation

• .venv/bin/python manage.py test inventory.tests.test_sale_status inventory.tests.test_sales_balance_payment inventory.tests.test_views.BackupViewSecurityTest
• .venv/bin/python manage.py test inventory.tests.test_i18n
• .venv/bin/python manage.py check