Add Shopify inventory sync and order webhook

- Migration: shopify_variant_id on products_sizes, shopify_order_id on products_sales
- ShopifyService: paginated REST client (2024-10 API)
- shopify:map — one-time command to link variants by SKU
- shopify:sync — reconciliation command (Shopify is master)
- VerifyShopifyWebhook middleware: HMAC-SHA256 verification
- ShopifyWebhookController: idempotent orders/paid handler, FIFO inventory deduction
- Webhook route: POST /api/webhooks/shopify/orders-paid

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

config/services.php

@@ -35,4 +35,11 @@ return [
         ],
     ],
 
+    'shopify' => [
+        'domain'         => env('SHOPIFY_SHOP_DOMAIN'),
+        'access_token'   => env('SHOPIFY_ACCESS_TOKEN'),
+        'webhook_secret' => env('SHOPIFY_WEBHOOK_SECRET'),
+        'api_version'    => '2024-10',
+    ],
+
 ];