Product From Account Move Line Link

This module adds a smart button to product forms so accounting users can jump straight from a product to the accounting entries that mention it.

• On both the product template form and the product variant form, a "Journal Items" stat button appears in the button box, showing a count labelled "Invoiced".
• Clicking the button opens a list of journal items (account.move.line) filtered to the product(s) you were viewing, with the search pre-set to posted entries.
• The count is computed automatically: for a product variant it counts journal items whose product is that variant and whose company is one of the companies currently selected by the user; for a product template it sums the counts of all its variants (including archived ones).
• The button and the count are only visible to users belonging to the invoicing/accounting group; other users see a count of zero.

It touches the product.product and product.template models by adding the computed "Invoiced" field, and reuses the standard journal items list view from the Accounting app. No new menus are created; the entry point is the button on the product form.