Fixes: 1) Correct `notifications` field handling in `OrderAdmin` and `OrderItemAdmin` save logic; 2) Fix admin `get_queryset` prefetching logic for various models.
Extra: Remove outdated labels and sections in fieldsets; clean up unused imports and commented code for readability.
Fixes: 1) Correct field type hints and add `# type: ignore` to suppress validation issues;
Extra: Refactor code for formatting consistency and readability.
