feat: rename payer_dni to transfer_payer_dni across purchase handling and update related tests

database/migrations/2026_07_12_000000_add_payer_dni_to_compras_table.php

@@ -9,14 +9,14 @@ return new class extends Migration
     public function up(): void
     {
         Schema::table('compras', function (Blueprint $table) {
-            $table->string('payer_dni', 8)->nullable()->after('dni');
+            $table->string('transfer_payer_dni', 8)->nullable()->after('dni');
         });
     }
 
     public function down(): void
     {
         Schema::table('compras', function (Blueprint $table) {
-            $table->dropColumn('payer_dni');
+            $table->dropColumn('transfer_payer_dni');
         });
     }
 };