fixes and check for valid cron expressions

database/migrations/2024_08_05_142659_add_new_version_available_to_instance_settings_table.php

@@ -12,8 +12,8 @@ return new class extends Migration
     public function up(): void
     {
         Schema::table('instance_settings', function (Blueprint $table) {
-            $table->string('update_check_frequency')->nullable();
-            $table->string('auto_update_frequency')->nullable();
+            $table->string('update_check_frequency')->default('0 */12 * * *')->nullable();
+            $table->string('auto_update_frequency')->default('0 0 * * *')->nullable();
         });
     }