Fix critical sale integrity and backup restore failures

[cursor]

Bug and impact

• Unsupported payment_method=credit from the sale UI could complete a sale, decrement inventory, and update member spend without collecting money or recording receivables.
• Draft sale items could be deleted via GET and without row locks, allowing accidental/CSRF-triggered inventory restoration and unsafe concurrent writes.
• Viewing a sale detail page could rewrite persisted historical sale totals on GET.
• Backup restore with media could fail after the database snapshot was loaded and after current media was partially removed, leaving database/media inconsistent; restore failure rendering also crashed due missing template context and the UI confirmation field was not accepted.

Root cause

• Sale payment methods were assigned directly from POST without validating against Sale.PAYMENT_METHODS, while the template exposed an unimplemented credit option.
• Sale item deletion used an unsafe GET link and non-atomic inventory mutation.
• Sale detail contained auto-repair SQL in a read path.
• Backup media restore copied into MEDIA_ROOT only after committing database restore, and failure renders did not supply the backup context expected by the template.

Fix

• Added payment method normalization/validation, retained legacy account -> balance, and removed the unimplemented credit button.
• Made sale item deletion POST-only with CSRF form rendering, transaction boundaries, and row locks.
• Removed sale-detail GET mutation of persisted amounts.
• Staged backup media before DB changes, swapped media with rollback support inside the restore failure path, accepted the template confirm_restore field, and supplied restore template context consistently.

Validation

• python3 manage.py test inventory.tests.test_sales_balance_payment inventory.tests.test_sale_status inventory.tests.test_views.BackupViewSecurityTest inventory.tests.test_services.MemberServiceTest
• Result: 17 tests OK; only existing staticfiles.W004 warning for missing /workspace/static.