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>

routes/api.php

@@ -1,8 +1,13 @@
 <?php
 
 use App\Http\Controllers\Api\LotController;
+use App\Http\Controllers\ShopifyWebhookController;
+use App\Http\Middleware\VerifyShopifyWebhook;
 use Illuminate\Support\Facades\Route;
 
 Route::get('/lot/{lot_number}', [LotController::class, 'show'])
     ->middleware('throttle:30,1')
     ->name('api.lot');
+
+Route::post('/webhooks/shopify/orders-paid', [ShopifyWebhookController::class, 'ordersPaid'])
+    ->middleware(VerifyShopifyWebhook::class);