Fix destructive sales and backup restore regressions

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Bug and impact

• Viewing a sale detail page could rewrite persisted sale totals on GET when line subtotals differed from the recorded paid amount. This corrupts historical receipts/member accounting without an explicit edit.
• Draft sale item deletion was reachable via GET and restored inventory without row locks, allowing accidental/CSRF-like inventory inflation and races with other sale state changes.
• The checkout UI exposed an unsupported credit payment method, and the backend accepted arbitrary payment values, so a sale could be marked completed without implemented receivables/payment handling.
• Backup restore could not be completed from the active template (confirm_restore/backup mismatch), and media restore failures after database load could leave the system partially restored.

Root cause

• Read-only sale detail code performed corrective SQL updates.
• Sale item deletion did not enforce POST or transactional row locking.
• Payment methods were copied from POST without normalizing/validating against backend-supported choices.
• Backup restore view/template contracts drifted, and media replacement occurred after the DB transaction without staging/rollback.

Fix

• Keep sale detail read-only.
• Require POST+CSRF for sale item deletion and lock sale/item/inventory rows while restoring stock and recalculating totals.
• Normalize legacy account to balance, reject unsupported payment methods, and replace the checkout credit button with supported other.
• Align backup restore context/confirmation fields, add a missing delete confirmation template/POST modal flow, and stage media before replacing it inside the DB restore transaction with rollback of old media on failure.
• Added regression tests for each critical trigger.

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_services.MemberServiceTest (19 tests OK; expected 405 and simulated media failure logs; existing staticfiles warning)
• mkdir -p logs && .venv/bin/python manage.py check (OK; existing staticfiles warning)