Subscription purchase prevention logic

🔍 Problem Statement
As a subscriber, I get no warning when I buy a second subscription to a product I already have, so I get charged twice.

💡 User Story

• As a subscriber, I want a warning before I buy a product I already subscribe to, so that I don’t pay twice for the same access.

• As a subscriber, I want a Renew option in that warning, so that I can extend my current subscription instead of buying a new one.

• As a publisher, I want to turn this check on per site, so that sites that sell several subscriptions per customer keep working.

🎯 Definition of Done (DoD)
✔ Given the setting is on and the customer has a current subscription to product A, when they start checkout for any plan of product A, then the prevention modal displays.
✔ Given the customer’s current subscriptions are all to other products, when they start checkout for product B, then checkout continues with no modal.
✔ A current subscription is active, in a free trial, or extended. Donations and gifts bought for someone else do not count.
✔ Given the modal displays, when the customer clicks Renew, then they go to the renewal checkout for their current subscription [clarify: renew on the plan they selected or the plan they have].
✔ Given the current subscription cannot be renewed, when the modal displays, then the Renew option is hidden.
✔ Given the modal displays, when the customer closes it, then no subscription or charge is created.
✔ Given the setting is on, when a purchase request for a product the customer already has reaches Pelcro by any route, then Pelcro blocks it.
✔ The setting is off by default, so existing sites keep today’s behavior.
✔ The setting and the new purchase rule are documented on docs.pelcro.com and covered by automated tests.