PFT MG (Polished Flat Top + Margin) base finish — allow same-size bases with different finishes

Add a PFT MG (Polished Flat Top + Margin) finish option when configuring a base in the new quote. Whenever a base is created that also has a polished margin, it becomes a distinct PFT MG base item.

Inventory should allow two bases of the same size but different finishes (e.g. 5-6x1-2x0-8 PFT and 5-6x1-2x0-8 PFT MG) instead of blocking the second one as a duplicate. The uniqueness key should be type + color + size + finish.

Requested by Luther (West KY Memorials).