ITA - Autofatture e date di competenza

This is a technical bridge module that combines two existing features: Italian reverse-charge self-invoices and the start/end competence dates on invoice lines. It does not add new menus or screens; it changes how existing buttons behave.

What it changes:

• When a user generates a supplier self-invoice (the reverse-charge self-invoice created from a supplier bill), the module temporarily disables the "must have dates" flag on the products involved, so the self-invoice can be posted even if those products normally require start and end dates.
• After the self-invoice is created, the start and end dates on its invoice lines are cleared, so the self-invoice does not carry competence dates copied from the original bill.
• The same temporary bypass is applied when generating a customer self-invoice.
• Once posting is finished, the "must have dates" flag is restored on the products, so normal behaviour is unchanged for other invoices.

It touches the account.move model and its posting and self-invoice generation methods. No new fields, menus, reports or scheduled actions are added.