You don't need to need to bulk-verify your entire database. A subscription is valid for a fixed duration. If the user cancels their subscription then this takes effect at the next renewal period, not immediately.
Apple customer service can refund a subscription if a user has a case (wrong product purchase is one example they give) but this would be a pretty low figure - and your app should also check for a valid subscription when it starts.
So you should only need to verify subscriptions that are known to be expiring "today".