Harden Celery schedule parsing and startup safety

README.md

@@ -296,6 +296,18 @@ Configure scheduled sync through environment variables:
 When enabled, Celery Beat enqueues the scheduled sync task on the configured cron.
 The task uses the existing ingestion service path and writes run/error records in the same tables as manual sync.
 
+Valid cron examples:
+
+- `*/30 * * * *` every 30 minutes
+- `0 * * * *` hourly
+- `15 2 * * *` daily at 02:15
+
+Failure behavior for invalid cron values:
+
+- invalid `INGESTION_SCHEDULE_CRON` does not crash unrelated startup paths (for example, web)
+- periodic ingestion task is disabled until cron is fixed
+- an error is logged at startup indicating the invalid schedule value
+
 ## Provider Backend Selection
 
 Provider backend is selected via environment variables: