Limit available subscriptions in Subi without Shopify inventory

Right now, Subi uses the same inventory as our Shopify product inventory, but we do not want to manage inventory on the product just to control subscription sales.

We want a Subi setting to limit how many subscriptions can be sold for a product or variant (a subscription-specific cap), even if Shopify inventory tracking is off or not used. This would help us avoid selling more subscriptions than we can fulfill for our customers.