fix(recurring-invoices): scale scheduled generation

app/Rules/ValidCronExpression.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace App\Rules;
+
+use Closure;
+use Cron\CronExpression;
+use Illuminate\Contracts\Validation\ValidationRule;
+use Illuminate\Translation\PotentiallyTranslatedString;
+
+class ValidCronExpression implements ValidationRule
+{
+    /**
+     * Run the validation rule.
+     *
+     * @param  Closure(string, ?string=): PotentiallyTranslatedString  $fail
+     */
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        if (! is_string($value) || ! CronExpression::isValidExpression($value)) {
+            $fail('The frequency must be a valid cron expression.');
+        }
+    }
+}