fix: 修正空白的 add_username_to_users_table migration 檔案

database/migrations/2026_01_12_031836_add_username_to_users_table.php

@@ -11,18 +11,22 @@ return new class extends Migration
      */
     public function up(): void
     {
+        if (!Schema::hasColumn('users', 'username')) {
             Schema::table('users', function (Blueprint $table) {
-            //
+                $table->string('username')->unique()->nullable()->after('id');
             });
         }
+    }
 
     /**
      * Reverse the migrations.
      */
     public function down(): void
     {
+        if (Schema::hasColumn('users', 'username')) {
             Schema::table('users', function (Blueprint $table) {
-            //
+                $table->dropColumn('username');
             });
         }
+    }
 };