Revert "remove --concurrency=1 --pool=solo"

This reverts commit 06a32a55.

Also delays validation workflows by three hours

gso/schedules/validate_subscriptions.py

@@ -13,7 +13,7 @@ logger = structlog.get_logger(__name__)
 
 
 @celery.task
-@scheduler(CronScheduleConfig(name="Subscriptions Validator", minute="10", hour="0"))
+@scheduler(CronScheduleConfig(name="Subscriptions Validator", minute="10", hour="3"))
 def validate_subscriptions() -> None:
     """Validate all subscriptions using their corresponding validation workflow."""
     subscriptions = get_active_insync_subscriptions()