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>

app/routes/vendor_pages.py

@@ -280,6 +280,25 @@ async def vendor_products_page(
     )
 
 
+@router.get(
+    "/{vendor_code}/products/create", response_class=HTMLResponse, include_in_schema=False
+)
+async def vendor_product_create_page(
+    request: Request,
+    vendor_code: str = Path(..., description="Vendor code"),
+    current_user: User = Depends(get_current_vendor_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render product creation page.
+    JavaScript handles form submission via API.
+    """
+    return templates.TemplateResponse(
+        "vendor/product-create.html",
+        get_vendor_context(request, db, current_user, vendor_code),
+    )
+
+
 # ============================================================================
 # ORDER MANAGEMENT
 # ============================================================================