Substance field on Products and Customer Invoice lines

We've added an optional substance field (goods or services) on Company API v2 products and customer invoice lines.

When provided, it resolves the ledger account to the company's goods or services account. If it conflicts with ledger_account_id (or, on invoice create, with the account inferred from product_id), the API returns 422.

On invoice import, substance is supported but product_id is not auto-filled or validated against it.

substance is also returned on product and invoice line responses (null when it cannot be determined).

Affected endpoints

• POST/PUT/GET /products
• POST /customer_invoices
• POST /customer_invoices/imports
• GET /customer_invoices/{id}/invoice_lines
• GET customer_invoices?include=invoice_lines