feat(catalog): add max_units_per_user field to catalog items and implement purchase limit validation

database/migrations/2026_08_06_000500_add_max_units_per_user_to_catalog_items.php

@@ -0,0 +1,22 @@
+<?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('catalog_items', function (Blueprint $table): void {
+            $table->unsignedInteger('max_units_per_user')->nullable();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('catalog_items', function (Blueprint $table): void {
+            $table->dropColumn('max_units_per_user');
+        });
+    }
+};