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/Models/ProductSize.php

@@ -9,7 +9,7 @@ class ProductSize extends Model
 {
     use HasFactory;
     protected $table = 'products_sizes';
-    protected $fillable = ['product_id', 'packing_id', 'sku', 'price', 'description'];
+    protected $fillable = ['product_id', 'packing_id', 'sku', 'shopify_variant_id', 'price', 'description'];
 
     public function product() {
         return $this->belongsTo(Product::class);