Fix critical sale status and backup restore regressions

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Bug and impact

• Completed/cancelled sales could still be modified through legacy item endpoints, causing duplicate inventory changes or unpaid additions to completed sales.
• Draft sale completion updated member stats before balance validation and did not lock member rows, allowing corrupted member spend/points on insufficient balance.
• Backup restore used loaddata without clearing existing rows, producing a mixed database state instead of the selected snapshot.
• Sale creation rechecked stock outside the transaction only, leaving a race window for overselling.

Root cause

• The new sale status field was not enforced across all legacy sale item/cancel/complete paths.
• The draft completion path did not use the same atomic balance handling as the one-step sale creation path.
• Django fixture loading upserts rows and does not remove rows absent from the fixture.
• Inventory rows were not locked and revalidated at the point of deduction.

Fix

• Restrict sale item create/delete and cancel/complete actions to valid sale statuses and make cancel idempotency-safe with row locks.
• Make draft sale completion atomic, lock sale/member rows, validate balances before stats writes, and record balance transactions.
• Add a cancel confirmation template and pass payment method choices explicitly to the complete template.
• Flush the database before loading a backup snapshot and skip restore log creation if the restoring user no longer exists after restore.
• Lock and recheck inventory rows inside the sale creation transaction before decrementing stock.

Validation

• Passed: python3 manage.py test inventory.tests.test_sale_status inventory.tests.test_sales_balance_payment inventory.tests.test_views.BackupViewSecurityTest inventory.tests.test_services.MemberServiceTest
• Broader related run: python3 manage.py test inventory.tests.test_sale_status inventory.tests.test_sales_balance_payment inventory.tests.test_services inventory.tests.test_models inventory.tests.test_integration still shows two pre-existing integration failures around legacy no-product sale creation and missing inventory-check permission; unrelated targeted coverage passed.