Extensión del contraste SII para el TPV

This module makes the SII matching report work for point-of-sale orders, so users can check whether the tickets and orders sent to the Spanish tax agency (AEAT) were actually received and whether the data matches.

On a point-of-sale order form, users with the AEAT accounting group see a "Contrast with AEAT" button. It only appears when the order has already been sent to the SII. Pressing it compares the order with the AEAT response; if the order was never sent, Odoo shows an error listing the orders that cannot be matched.

The order form also shows the SII match state, the AEAT contrast state, the values sent versus the values returned, and a list of differences found (field, value returned by AEAT, value sent). In the point-of-sale orders list, users can group orders by "SII match state" or "AEAT contrast state". A server action named "Contrast Invoices with AEAT" is also available from the orders list.

The matching report itself is extended so that, for outgoing documents, it looks for point-of-sale orders by their SII CSV code or by their unique invoice number, and it also lists orders that were sent to the SII but do not appear in the AEAT response. Incoming documents are left unchanged.