feat: set market_place_id on CartItem when adding to cart (Phase 3)

Track which market a cart item came from by setting market_place_id
from g.market.id on new CartItem creation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/product/routes.py

@@ -221,6 +221,7 @@ def register():
                     product_id=product.id,
                     product=product,
                     quantity=count,
+                    market_place_id=getattr(g, "market", None) and g.market.id,
                 )
                 g.cart.append(ci)
                 g.s.add(ci)