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/Services/ShopifyService.php

@@ -2,6 +2,7 @@
 
 namespace App\Services;
 
+use App\Models\ProductSize;
 use Illuminate\Support\Facades\Http;
 use Illuminate\Support\Facades\Log;
 
@@ -51,11 +52,12 @@ class ShopifyService
                 $imageUrls = collect($product['images'] ?? [])->pluck('src')->all();
                 foreach ($product['variants'] as $variant) {
                     $variants[] = [
-                        'id'                 => (string) $variant['id'],
-                        'sku'                => $variant['sku'] ?? '',
-                        'price'              => (float) ($variant['price'] ?? 0),
+                        'id'                  => (string) $variant['id'],
+                        'sku'                 => $variant['sku'] ?? '',
+                        'price'               => (float) ($variant['price'] ?? 0),
                         'inventory_quantity'  => (int) ($variant['inventory_quantity'] ?? 0),
                         'product_title'       => $product['title'],
+                        'shopify_product_id'  => (string) $product['id'],
                         'product_description' => strip_tags($product['body_html'] ?? ''),
                         'option1'             => $variant['option1'] ?? '',
                         'image_urls'          => $imageUrls,
@@ -74,6 +76,49 @@ class ShopifyService
         return $variants;
     }
 
+    public function createVariant(string $shopifyProductId, ProductSize $size): string
+    {
+        $packing  = $size->packing;
+        $option1  = $packing->name . ' ' . $packing->size . 'g';
+
+        $response = Http::withHeader('X-Shopify-Access-Token', $this->token)
+            ->post("{$this->baseUrl}/products/{$shopifyProductId}/variants.json", [
+                'variant' => array_filter([
+                    'option1' => $option1,
+                    'price'   => number_format((float) $size->price, 2, '.', ''),
+                    'sku'     => $size->sku ?: null,
+                    'barcode' => $size->gtin ?: null,
+                ]),
+            ]);
+
+        if ($response->failed()) {
+            throw new \RuntimeException(
+                'Shopify-Fehler ' . $response->status() . ': ' . $response->body()
+            );
+        }
+
+        return (string) $response->json('variant.id');
+    }
+
+    public function updateVariant(ProductSize $size): void
+    {
+        $response = Http::withHeader('X-Shopify-Access-Token', $this->token)
+            ->put("{$this->baseUrl}/variants/{$size->shopify_variant_id}.json", [
+                'variant' => array_filter([
+                    'id'      => $size->shopify_variant_id,
+                    'price'   => number_format((float) $size->price, 2, '.', ''),
+                    'sku'     => $size->sku ?: null,
+                    'barcode' => $size->gtin ?: null,
+                ], fn ($v) => $v !== null),
+            ]);
+
+        if ($response->failed()) {
+            throw new \RuntimeException(
+                'Shopify-Fehler ' . $response->status() . ': ' . $response->body()
+            );
+        }
+    }
+
     private function nextPageUrl(?string $linkHeader): ?string
     {
         if (!$linkHeader) {