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/api/v1/admin/vendor_products.py

@@ -131,8 +131,14 @@ class VendorProductDetail(BaseModel):
     # Supplier tracking
     supplier: str | None = None
     supplier_product_id: str | None = None
-    supplier_cost: float | None = None
+    cost: float | None = None  # What vendor pays to acquire product
     margin_percent: float | None = None
+    # Tax/profit info
+    tax_rate_percent: int | None = None
+    net_price: float | None = None
+    vat_amount: float | None = None
+    profit: float | None = None
+    profit_margin_percent: float | None = None
     # Digital fulfillment
     download_url: str | None = None
     license_type: str | None = None