Fix unsafe sales payment and deletion paths

[cursor]

Bug and impact

• The checkout UI exposed an unsupported credit payment method, and the backend persisted arbitrary payment strings without model/form validation. Selecting it completed a sale and decremented inventory without recording any supported payment, balance deduction, or receivable.
• Viewing a sale detail page rewrote persisted historical sale totals as a GET side effect, which could corrupt completed-sale/payment history.
• Draft sale item deletion was reachable via GET links, so navigation/prefetch/CSRF could delete line items and restock inventory.

Root cause

• payment_method was assigned directly in sales views while SaleForm only validates remark.
• sale_detail contained repair SQL in the read path.
• sale_delete_item accepted all methods and updated sale/item/inventory without row locks.

Fix

• Normalize legacy account to balance and reject unsupported payment methods before saving.
• Remove the unsupported credit button from checkout.
• Make sale detail read-only.
• Require POST+CSRF for item deletion and perform deletion/restock/total updates inside a transaction with row locks.
• Remove the hardcoded fallback sale amount.

Validation

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

Both passed; only existing staticfiles.W004 warning was reported.