feat: 新增商品 Excel 匯入功能與修復 HTTPS 混合內容問題

1. 新增商品 Excel 匯入功能 (ProductImport, Export Template)
2. 調整商品代號驗證規則為 1-5 碼 (Controller & Import)
3. 修正 HTTPS Mixed Content 問題 (AppServiceProvider)

app/Modules/Inventory/Routes/web.php

@@ -30,6 +30,8 @@ Route::middleware('auth')->group(function () {
 
     // 商品管理
     Route::middleware('permission:products.view')->group(function () {
+        Route::get('/products/template', [ProductController::class, 'template'])->name('products.template');
+        Route::post('/products/import', [ProductController::class, 'import'])->middleware('permission:products.create')->name('products.import');
         Route::get('/products', [ProductController::class, 'index'])->name('products.index');
         Route::post('/products', [ProductController::class, 'store'])->middleware('permission:products.create')->name('products.store');
         Route::put('/products/{product}', [ProductController::class, 'update'])->middleware('permission:products.edit')->name('products.update');