Purchase order line price history discount

This module extends the purchase price history wizard so that the discount applied on each historical purchase line is visible and reusable.

• In the wizard that lists past prices for a purchase order line, a new Discount column appears next to the unit price, both in the list and in the detailed form of each history line.
• The discount shown is taken directly from the original purchase order line, so it always matches what was actually agreed with the vendor.
• When a user picks a historical line to bring its values back into the current purchase order line, the discount is copied along with the price, not just the unit price.

It touches the purchase price history line model (purchase.order.line.price.history.line) and the wizard view of purchase.order.line.price.history. No new menus or reports are added; the change is visible inside the existing price history wizard opened from a purchase order line. No scheduled jobs, emails or extra validation rules are introduced.