feat(integration): 擴充產品同步 API 欄位與驗證強化

1. ProductSync API 新增防護機制，為既有欄位加上字串長度與金額上限限制
2. 開放並接收 ERP Product Model 實用欄位（品牌、規格、成本價、會員價、批發價）
3. 更新 ProductService 寫入邏輯以支援新增的可選欄位
4. 同步更新 api-integration.md 手冊，加入新欄位說明與 JSON 範例

app/Modules/Integration/Controllers/ProductSyncController.php

@@ -19,12 +19,17 @@ class ProductSyncController extends Controller
     public function upsert(Request $request)
     {
         $request->validate([
-            'external_pos_id' => 'required|string',
-            'name' => 'required|string',
-            'price' => 'nullable|numeric|min:0',
-            'barcode' => 'nullable|string',
-            'category' => 'nullable|string',
-            'unit' => 'nullable|string',
+            'external_pos_id' => 'required|string|max:255',
+            'name' => 'required|string|max:255',
+            'price' => 'nullable|numeric|min:0|max:99999999.99',
+            'barcode' => 'nullable|string|max:100',
+            'category' => 'nullable|string|max:100',
+            'unit' => 'nullable|string|max:100',
+            'brand' => 'nullable|string|max:100',
+            'specification' => 'nullable|string|max:255',
+            'cost_price' => 'nullable|numeric|min:0|max:99999999.99',
+            'member_price' => 'nullable|numeric|min:0|max:99999999.99',
+            'wholesale_price' => 'nullable|numeric|min:0|max:99999999.99',
             'updated_at' => 'nullable|date',
         ]);