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>

database/migrations/2026_09_11_000001_shopify_variant_ids.php

@@ -0,0 +1,33 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('products_sizes', function (Blueprint $table) {
+            $table->string('shopify_variant_id')->nullable()->unique()->after('sku');
+        });
+
+        Schema::table('products_sales', function (Blueprint $table) {
+            $table->string('shopify_order_id')->nullable()->after('sold_at');
+            $table->index('shopify_order_id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('products_sales', function (Blueprint $table) {
+            $table->dropIndex(['shopify_order_id']);
+            $table->dropColumn('shopify_order_id');
+        });
+
+        Schema::table('products_sizes', function (Blueprint $table) {
+            $table->dropUnique(['shopify_variant_id']);
+            $table->dropColumn('shopify_variant_id');
+        });
+    }
+};