Fix critical sales and log integrity bugs

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Bug and impact

• Sale detail GET rewrote persisted sale totals from line items, corrupting completed/historical sales amounts and reports.
• Checkout exposed/accepted unsupported credit and one-step mixed payments, allowing completed sales without receivables or balance settlement.
• Draft sale item deletion was a GET side effect and non-atomic, allowing accidental/CSRF-triggered stock restoration and item deletion.
• Log file download/delete audit entries used content_type_id=0, causing FK failures after file operations.

Root cause

• Read-only and destructive sales routes lacked method/transaction boundaries and payment-method validation.
• Log file audit records used an invalid content type instead of nullable admin log content types.

Fix

• Made sale detail read-only, removed invalid amount fallback, normalized/validated payment methods, and replaced the unsupported credit button with other.
• Restricted sale item deletion to POST with CSRF template form and row locks.
• Changed log file audit entries to nullable content types and added missing delete confirmation template.

Validation

• mkdir -p /workspace/logs && .venv/bin/python manage.py test inventory.tests.test_sale_status inventory.tests.test_sales_balance_payment inventory.tests.test_views.LogFileViewTest (17 tests OK)
• .venv/bin/python manage.py check (only existing staticfiles.W004 warning)