Fix destructive sale read and item deletion paths

[cursor]

Bug and impact

• Viewing a sale detail page could rewrite persisted sale totals from line-item subtotals. For completed historical sales this can corrupt reported revenue and leave member balances/transactions inconsistent with the sale amount.
• The draft sale item delete endpoint accepted GET requests and performed inventory restoration/deletion without row locks. A browser prefetch, copied link, or CSRF-style GET could delete a draft line and inflate stock.

Root cause

• sale_detail performed repair writes during a read-only GET.
• sale_delete_item did not require POST/CSRF and updated inventory/item/sale rows outside a transaction with row locks.

Fix

• Made sale detail read-only.
• Restricted sale item deletion to POST and changed the template delete control to a CSRF-protected form.
• Wrapped deletion/restock/recalculation in a transaction with row locks.
• Added regression tests for GET no-op deletion and sale detail amount preservation.

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 (15 tests OK; only existing staticfiles.W004 warning).