fix: validate cron expression before writing to crontab

api/crontab_manager.py

@@ -1,5 +1,5 @@
 import uuid
-from crontab import CronTab
+from crontab import CronTab, CronSlices
 
 
 class CrontabManager:
@@ -39,6 +39,9 @@ class CrontabManager:
         command: str,
         is_enabled: bool = True,
     ):
+        if not CronSlices.is_valid(cron_expression):
+            raise ValueError(f"Invalid cron expression: {cron_expression!r}")
+
         if not alarm_id:
             alarm_id = str(uuid.uuid4())