Connector name: Kantata OX

Table to add: ADDITIONAL_ITEM (with invoice_id as foreign key
to the INVOICE table)

API documentation link:
https://developer.kantata.com/kantata/specification/invoices/get-invoice

Context: Additional Items are one of the four line item types that
make up an invoice in Kantata OX — alongside time entries,
expenses and fixed fee items. The Kantata documentation states
that an invoice must have at least one line item of one of these
four types.

The data is already reachable through the existing Invoices
endpoint: passing include=additional_items on GET /invoices
returns the additional_item_ids array referencing all additional
items on that invoice. The connector currently syncs invoices but
not this association, which leaves our invoice data structurally
incomplete — we cannot reconstruct an invoice total from the
line items we receive.

This blocks an automated invoicing workflow we are building on
top of the Kantata connector.

We would also request a historical backfill once the table is
added.