feat: make Product fully independent from MarketplaceProduct

- Add is_digital and product_type columns to Product model
- Remove is_digital/product_type properties that derived from MarketplaceProduct
- Update Create form with translation tabs, GTIN type, sale price, VAT rate, image
- Update Edit form to allow editing is_digital (remove disabled state)
- Add Availability field to Edit form
- Fix Detail page for directly created products (no marketplace source)
- Update vendor_product_service to handle new fields in create/update
- Add VendorProductCreate/Update schema fields for translations and is_digital
- Add unit tests for is_digital column and direct product creation
- Add integration tests for create/update API with new fields
- Create product-architecture.md documenting the independent copy pattern
- Add migration y3d4e5f6g7h8 for is_digital and product_type columns

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

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

mkdocs.yml

@@ -34,6 +34,7 @@ nav:
       - Architecture Patterns: architecture/architecture-patterns.md
       - Multi-Tenant System: architecture/multi-tenant.md
       - Marketplace Integration: architecture/marketplace-integration.md
+      - Product Architecture: architecture/product-architecture.md
       - Language & i18n: architecture/language-i18n.md
       - Money Handling: architecture/money-handling.md
       - Company-Vendor Management: architecture/company-vendor-management.md