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>

static/admin/js/vendor-product-edit.js

@@ -59,6 +59,7 @@ function adminVendorProductEdit() {
             sale_price: null,
             currency: 'EUR',
             tax_rate_percent: 17,
+            availability: '',
             // Image
             primary_image_url: '',
             // Product type & status
@@ -125,6 +126,7 @@ function adminVendorProductEdit() {
                     sale_price: response.sale_price || null,
                     currency: response.currency || 'EUR',
                     tax_rate_percent: response.tax_rate_percent ?? 17,
+                    availability: response.availability || '',
                     // Image
                     primary_image_url: response.primary_image_url || '',
                     // Product type & status
@@ -238,9 +240,11 @@ function adminVendorProductEdit() {
                     currency: this.form.currency || null,
                     tax_rate_percent: this.form.tax_rate_percent !== null && this.form.tax_rate_percent !== ''
                         ? parseInt(this.form.tax_rate_percent) : null,
+                    availability: this.form.availability || null,
                     // Image
                     primary_image_url: this.form.primary_image_url?.trim() || null,
-                    // Status (is_digital is derived from marketplace product, not editable)
+                    // Status
+                    is_digital: this.form.is_digital,
                     is_active: this.form.is_active,
                     is_featured: this.form.is_featured,
                     // Optional supplier info