fix(vendor): show company values in business details placeholders

Added company_business_address and company_tax_number to:
- VendorDetailResponse schema
- API response builder

Template now shows actual company values as placeholders instead of
generic "Using company address/tax number" text.

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

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

app/api/v1/admin/vendors.py

@@ -151,6 +151,8 @@ def _build_vendor_detail_response(vendor) -> VendorDetailResponse:
         company_contact_email=vendor.company.contact_email,
         company_contact_phone=vendor.company.contact_phone,
         company_website=vendor.company.website,
+        company_business_address=vendor.company.business_address,
+        company_tax_number=vendor.company.tax_number,
     )