feat: add ProductVariant model and service layer for CRUD operations with attachment handling

database/migrations/2026_07_01_100000_add_is_placeholder_to_productos_variantes_table.php

@@ -12,7 +12,7 @@ return new class extends Migration
     public function up(): void
     {
         Schema::table('productos_variantes', function (Blueprint $table) {
-            $table->boolean('is_default')->default(false)->after('stock');
+            $table->boolean('is_placeholder')->default(false)->after('stock');
         });
     }
 
@@ -22,7 +22,7 @@ return new class extends Migration
     public function down(): void
     {
         Schema::table('productos_variantes', function (Blueprint $table) {
-            $table->dropColumn('is_default');
+            $table->dropColumn('is_placeholder');
         });
     }
 };