feat: add is_default field to product variants and implement default variant management logic in ProductService

database/migrations/2026_07_01_100000_add_is_default_to_productos_variantes_table.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('productos_variantes', function (Blueprint $table) {
+            $table->boolean('is_default')->default(false)->after('stock');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('productos_variantes', function (Blueprint $table) {
+            $table->dropColumn('is_default');
+        });
+    }
+};