Fix syncSizes: replace belongsToMany sync with ProductSize updateOrCreate

belongsToMany sync() is wrong for products_sizes which is a full model
with its own id and a unique shopify_variant_id. The new approach:
- updateOrCreate per row (safe for repeated saves)
- strips non-digits from gtin, stores null when empty (fixes 500 on
  unsignedBigInteger column with empty string in strict SQL mode)
- only deletes local-only sizes (whereNull shopify_variant_id) that
  were removed — Shopify-imported sizes are never deleted here

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

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

@@ -3,6 +3,7 @@
 namespace App\Filament\User\Resources\ProductResource\Pages;
 
 use App\Filament\User\Resources\ProductResource;
+use App\Models\ProductSize;
 use App\Models\Recipe;
 use Filament\Actions;
 use Filament\Resources\Pages\EditRecord;
@@ -31,22 +32,38 @@ class EditProduct extends EditRecord
 
     protected function syncSizes(): void
     {
-        $product = $this->record;
+        $product     = $this->record;
+        $incoming    = collect($this->data['sizes'] ?? []);
+        $keepIds     = [];
 
-        $sizes = collect($this->data['sizes'] ?? [])
-            ->mapWithKeys(function ($size) {
-                return [
-                    $size['packing_id'] => [
-                        'description' => $size['description'] ?? null,
-                        'price'       => $size['price'],
-                        'sku'         => $size['sku'] ?? null,
-                        'gtin'        => $size['gtin'] ?? null,
-                    ],
-                ];
-            })
-            ->toArray();
+        foreach ($incoming as $size) {
+            if (empty($size['packing_id'])) {
+                continue;
+            }
 
-        $product->sizes()->sync($sizes);
+            $gtin = preg_replace('/\D/', '', (string) ($size['gtin'] ?? ''));
+
+            $ps = ProductSize::updateOrCreate(
+                [
+                    'product_id' => $product->id,
+                    'packing_id' => (int) $size['packing_id'],
+                ],
+                [
+                    'price'       => $size['price'],
+                    'sku'         => $size['sku'] ?: null,
+                    'gtin'        => $gtin !== '' ? $gtin : null,
+                    'description' => $size['description'] ?? null,
+                ]
+            );
+
+            $keepIds[] = $ps->id;
+        }
+
+        // Only delete local-only sizes (no shopify_variant_id) that were removed
+        ProductSize::where('product_id', $product->id)
+            ->whereNotIn('id', $keepIds)
+            ->whereNull('shopify_variant_id')
+            ->delete();
     }
 
     protected function syncRecipe(): void