Add Shopify publish/sync per product variant

Each size row in the product form now shows a Shopify status badge
(green ✓ Shopify / gray Lokal). For saved variants, an action button
appears: 'Zu Shopify' for local-only variants (creates via Shopify
API and stores the returned variant ID), 'Shopify sync' for linked
variants (pushes price/SKU/GTIN to Shopify).

Also fixes:
- gtin missing from ProductSize::$fillable (was silently not saved)
- shopify_product_id added to products table and stored on import
- sizes afterStateHydrated now loads ProductSize directly instead of
  via the belongsToMany pivot (gets product_size_id, shopify_variant_id)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Filament/User/Resources/ProductResource/Pages/EditProduct.php

@@ -5,7 +5,9 @@ namespace App\Filament\User\Resources\ProductResource\Pages;
 use App\Filament\User\Resources\ProductResource;
 use App\Models\ProductSize;
 use App\Models\Recipe;
+use App\Services\ShopifyService;
 use Filament\Actions;
+use Filament\Notifications\Notification;
 use Filament\Resources\Pages\EditRecord;
 
 class EditProduct extends EditRecord
@@ -109,4 +111,46 @@ class EditProduct extends EditRecord
 
         $product->update(['recipes_id' => $recipe->id]);
     }
+
+    public function publishVariantToShopify(int $sizeId): void
+    {
+        $size    = ProductSize::with('packing')->find($sizeId);
+        $product = $this->record;
+
+        if (! $size || $size->shopify_variant_id || ! $product->shopify_product_id) {
+            Notification::make()
+                ->title('Nicht möglich')
+                ->body($product->shopify_product_id
+                    ? 'Variante bereits in Shopify verknüpft.'
+                    : 'Produkt hat keine Shopify-ID — zuerst shopify:import ausführen.')
+                ->warning()
+                ->send();
+            return;
+        }
+
+        try {
+            $variantId = app(ShopifyService::class)->createVariant($product->shopify_product_id, $size);
+            $size->update(['shopify_variant_id' => $variantId]);
+            Notification::make()->title('Variante in Shopify veröffentlicht')->success()->send();
+        } catch (\Throwable $e) {
+            Notification::make()->title('Shopify-Fehler')->body($e->getMessage())->danger()->send();
+        }
+    }
+
+    public function syncVariantWithShopify(int $sizeId): void
+    {
+        $size = ProductSize::find($sizeId);
+
+        if (! $size || ! $size->shopify_variant_id) {
+            Notification::make()->title('Keine Shopify-Verknüpfung')->warning()->send();
+            return;
+        }
+
+        try {
+            app(ShopifyService::class)->updateVariant($size);
+            Notification::make()->title('Shopify synchronisiert')->success()->send();
+        } catch (\Throwable $e) {
+            Notification::make()->title('Shopify-Fehler')->body($e->getMessage())->danger()->send();
+        }
+    }
 }