Fix critical sales and backup restore regressions

[cursor]

Summary

• Prevent sale detail GET requests from rewriting historical sale amounts.
• Reject unsupported sale payment methods and remove the exposed credit checkout option.
• Require POST plus row locks for sale item deletion.
• Make backup restore confirmation/template context consistent and roll back DB/media changes if media replacement fails.
• Add minimal system templates/URLs needed by the restore/settings flow.

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
• mkdir -p logs && .venv/bin/python manage.py test inventory.tests.test_i18n inventory.tests.test_services.MemberServiceTest
• mkdir -p logs && .venv/bin/python manage.py check