feat: add payer_dni field and validation for transfer payments, update related logic and tests

database/migrations/2026_07_12_000000_add_payer_dni_to_compras_table.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('compras', function (Blueprint $table) {
+            $table->string('payer_dni', 8)->nullable()->after('dni');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('compras', function (Blueprint $table) {
+            $table->dropColumn('payer_dni');
+        });
+    }
+};