TicketBAI - OSS

This module connects the Spanish TicketBAI e-invoicing system with the European One-Stop Shop (OSS) rules. When a company sells to private consumers in other EU countries and uses OSS, the invoice must still be sent to TicketBAI, but with specific tax information.

• It adds a new VAT regime key, code 17, described as "Operación acogida a alguno de los regímenes previstos en el Capítulo XI del Título IX (OSS e IOSS)", available for selection on TicketBAI invoices.
• It adds a new non-subject cause "IE" (not subject in the Basque tax authority due to localisation rules, but foreign tax such as IPS/IGIC or another EU member state's VAT is charged).
• It automatically marks taxes configured with an OSS country as not subject to Basque tax for TicketBAI purposes, and treats them as "entrega" (supply) for the invoice breakdown.
• When an invoice uses a fiscal position flagged for OSS, the non-subject cause is automatically set to IE.
• It validates that TicketBAI invoices have a valid VAT regime key, raising an error if missing or invalid.

The module extends tbai.invoice, tbai.invoice.tax and account.tax. It does not add new menus or screens; the changes appear in the existing TicketBAI invoice and tax configuration forms.