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>

app/Http/Middleware/VerifyShopifyWebhook.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace App\Http\Middleware;
+
+use Closure;
+use Illuminate\Http\Request;
+use Symfony\Component\HttpFoundation\Response;
+
+class VerifyShopifyWebhook
+{
+    public function handle(Request $request, Closure $next): Response
+    {
+        $hmac   = $request->header('X-Shopify-Hmac-Sha256');
+        $secret = config('services.shopify.webhook_secret');
+
+        if (!$hmac || !$secret) {
+            abort(401);
+        }
+
+        $computed = base64_encode(hash_hmac('sha256', $request->getContent(), $secret, true));
+
+        if (!hash_equals($computed, $hmac)) {
+            abort(401);
+        }
+
+        return $next($request);
+    }
+}