feat: add attachment relationship to ProductVariant model and database schema

database/migrations/2026_06_29_000000_create_variantes_attachments_table.php

@@ -0,0 +1,35 @@
+<?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::create('variantes_attachments', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('variante_id')
+                ->constrained('productos_variantes')
+                ->cascadeOnDelete();
+            $table->foreignId('attachment_id')
+                ->constrained('attachments')
+                ->cascadeOnDelete();
+            $table->timestamps();
+
+            $table->unique(['variante_id', 'attachment_id']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('variantes_attachments');
+    }
+};