Free tickets no invoiceable

This module keeps the list of sales orders waiting to be invoiced clean by ignoring event ticket lines that cost nothing.

• When a sales order line is linked to an event ticket and its unit price is zero, the quantity to invoice is automatically set to 0. This means the line is treated as nothing to invoice, even though it is a real ticket line.
• The rule is applied every time Odoo recalculates the quantity to invoice on sale.order.line, so it also works for new orders created after installation.
• On installation, the module also cleans up existing data: it looks for sales order lines that have an event ticket, a price of 0 and a status of 'to invoice', and sets their quantity to invoice to 0.
• The check uses Odoo's standard rounding precision for product units of measure, so tiny amounts that round to zero are treated as free.
• It only touches sale.order.line; no new menus, screens or reports are added. The effect is visible in the invoicing status of sales orders and in the list of orders to invoice.

This is useful for events with free tickets: those lines stay on the order for tracking attendance, but they no longer clutter the invoicing workflow.