Fix backup restore media rollback and log audits

[cursor]

Bug and impact

• Backup restore committed DB changes before replacing media files; if media copy failed, restored DB state could be left paired with deleted or partially restored media.
• Log file download/delete used content_type_id=0, causing admin log IntegrityError; delete could remove the log file while reporting failure and recording no audit entry.
• Backup restore/delete UI paths were mismatched or missing templates, blocking normal admin recovery/deletion flows.

Root cause

• Media restore was performed after the DB transaction and destructively cleared MEDIA_ROOT before copy success was known.
• Custom admin log entries used an invalid content type FK in log views.
• Restore template submitted confirm_restore while the view required confirm, and delete referenced a missing confirmation template / GET link flow.

Fix

• Stage backup media first, restore DB and replace media in one transaction block, and restore prior media on any failure.
• Use nullable admin log content types for log file download/delete entries.
• Align restore confirmation handling, add the backup delete confirmation template, and make the backup-list modal submit a CSRF-protected POST.

Validation

• python manage.py test inventory.tests.test_views.BackupViewSecurityTest inventory.tests.test_views.LogFileViewTest
• python manage.py test inventory.tests.test_sales_balance_payment inventory.tests.test_sale_status inventory.tests.test_services.MemberServiceTest