refactor: 移除 SKU 欄位，統一使用 code 作為商品代碼

app/Modules/Integration/Controllers/ProductSyncController.php

@@ -22,7 +22,6 @@ class ProductSyncController extends Controller
             'external_pos_id' => 'required|string',
             'name' => 'required|string',
             'price' => 'nullable|numeric',
-            'sku' => 'nullable|string',
             'barcode' => 'nullable|string',
             'category' => 'nullable|string',
             'unit' => 'nullable|string',

app/Modules/Inventory/Models/Product.php

@@ -18,7 +18,6 @@ class Product extends Model
     protected $fillable = [
         'code',
         'barcode',
-        'sku',
         'name',
         'external_pos_id',
         'category_id',

app/Modules/Inventory/Services/ProductService.php

@@ -38,12 +38,11 @@ class ProductService
             // Map allowed fields
             $product->name = $data['name'];
             $product->barcode = $data['barcode'] ?? $product->barcode;
-            $product->sku = $data['sku'] ?? $product->sku; // Maybe allow SKU update?
             $product->price = $data['price'] ?? 0;
             
-            // Generate Code if missing (use sku or external_id)
+            // Generate Code if missing (use code or external_id)
             if (empty($product->code)) {
-                $product->code = $data['code'] ?? ($product->sku ?? $product->external_pos_id);
+                $product->code = $data['code'] ?? $product->external_pos_id;
             }
             
             // Handle Category (Default: 未分類)