fix: rename supplier_cost to cost and add tax/profit fields to product detail API

- Renamed supplier_cost to cost in VendorProductDetail schema
- Added tax_rate_percent, net_price, vat_amount, profit, profit_margin_percent
- Updated vendor_product_service to use new property names

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/vendor_product_service.py

@@ -193,8 +193,14 @@ class VendorProductService:
             # Supplier tracking
             "supplier": product.supplier,
             "supplier_product_id": product.supplier_product_id,
-            "supplier_cost": product.supplier_cost,
+            "cost": product.cost,
             "margin_percent": product.margin_percent,
+            # Tax/profit info
+            "tax_rate_percent": product.tax_rate_percent,
+            "net_price": product.net_price,
+            "vat_amount": product.vat_amount,
+            "profit": product.profit,
+            "profit_margin_percent": product.profit_margin_percent,
             # Digital fulfillment
             "download_url": product.download_url,
             "license_type": product.license_type,