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/templates/admin/vendor-product-edit.html

@@ -146,7 +146,7 @@
             <h3 class="mb-4 text-lg font-semibold text-gray-700 dark:text-gray-200">
                 Pricing
             </h3>
-            <div class="grid gap-4 md:grid-cols-2 lg:grid-cols-4">
+            <div class="grid gap-4 md:grid-cols-2 lg:grid-cols-5">
                 {# noqa: FE-008 - Using raw number input for price with EUR prefix #}
                 <div>
                     <label class="block text-sm font-medium text-gray-700 dark:text-gray-400 mb-1">
@@ -210,6 +210,21 @@
                         <option value="3">3% (Super-reduced)</option>
                     </select>
                 </div>
+                <div>
+                    <label class="block text-sm font-medium text-gray-700 dark:text-gray-400 mb-1">
+                        Availability
+                    </label>
+                    <select
+                        x-model="form.availability"
+                        class="w-full px-3 py-2 text-sm border border-gray-300 dark:border-gray-600 rounded-lg focus:border-purple-400 focus:outline-none dark:bg-gray-700 dark:text-gray-300"
+                    >
+                        <option value="">Not specified</option>
+                        <option value="in_stock">In Stock</option>
+                        <option value="out_of_stock">Out of Stock</option>
+                        <option value="preorder">Preorder</option>
+                        <option value="backorder">Backorder</option>
+                    </select>
+                </div>
             </div>
         </div>
 
@@ -253,14 +268,13 @@
                 Product Type & Status
             </h3>
             <div class="flex flex-wrap gap-6">
-                <label class="flex items-center opacity-60 cursor-not-allowed" title="Derived from marketplace product">
+                <label class="flex items-center">
                     <input
                         type="checkbox"
                         x-model="form.is_digital"
-                        disabled
-                        class="w-4 h-4 text-purple-600 border-gray-300 rounded focus:ring-purple-500 cursor-not-allowed"
+                        class="w-4 h-4 text-purple-600 border-gray-300 rounded focus:ring-purple-500"
                     />
-                    <span class="ml-2 text-sm text-gray-700 dark:text-gray-300">Digital Product <span class="text-xs text-gray-500">(from source)</span></span>
+                    <span class="ml-2 text-sm text-gray-700 dark:text-gray-300">Digital Product</span>
                 </label>
                 <label class="flex items-center">
                     <input