Line items are the catalog entries used to build invoices, one per billable product or service.

Each line item has a name, description, SKU, unit price, unit type, and tax rate.
A line item can optionally be assigned to a category to organize the catalog.
Unit types describe how each item is measured, such as by hour, unit, or pound.

The Line Items endpoints support creating, listing, updating, and deleting catalog entries.
The list endpoint accepts a search string and can filter by category or by items with no category.
A separate endpoint retrieves the available unit types a line item can reference.
Deleting a line item removes it from the catalog for future invoices.

* GET /pay-int-api/line-items.
Lists line items, filtered by search string, category, or items with no category.
* POST /pay-int-api/line-items.
Creates a new line item.
* PUT /pay-int-api/line-items/{lineItemId}.
Updates a specified line item.
* DELETE /pay-int-api/line-items/{lineItemId}.
Deletes a line item.
* GET /pay-int-api/line-items/unit-types.
Retrieves the available unit types for line items.