feat(catalog): implement maximum addable quantity logic and user quota sharing across variants

app/Domains/Catalog/Services/CatalogItemAllowanceService.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace App\Domains\Catalog\Services;
+
+use App\Domains\Catalog\Models\CatalogItem;
+use App\Domains\Purchase\Services\UserPurchaseLimitService;
+use Illuminate\Support\Collection;
+
+class CatalogItemAllowanceService
+{
+    public function __construct(
+        private readonly UserPurchaseLimitService $purchaseLimits,
+    ) {}
+
+    /** @param Collection<int, CatalogItem> $catalogItems */
+    public function attach(Collection $catalogItems, ?int $userId): void
+    {
+        $remaining = $this->purchaseLimits->remainingByCatalogItem($catalogItems, $userId);
+
+        foreach ($catalogItems as $catalogItem) {
+            $catalogItem->setAttribute(
+                'remaining_user_quota',
+                $remaining->get($catalogItem->getKey()),
+            );
+        }
+    }
+
+    public function maximumAddableQuantity(?int $availableStock, ?int $remainingUserQuota): ?int
+    {
+        if ($availableStock === null) {
+            return $remainingUserQuota;
+        }
+
+        if ($remainingUserQuota === null) {
+            return $availableStock;
+        }
+
+        return min($availableStock, $remainingUserQuota);
+    }
+}