Stock Notification Plugin for DOI.

Initial Subscription:

• If a guest registers for an unavailable product with their email address, they receive a Double Opt-In (DOI) request via email.
• Once the DOI is confirmed, they are notified via email when the product becomes available.

Re-Subscription to the Same Product:

• If the user registers for the same product again, a new DOI email is not sent if the email entry still exists in the database.

Database Entry Logic:

• The user entry in the table remains unless all subscriptions for the user are cleared.
• If the user entry is deleted (e.g., after all subscriptions are removed), they will receive a DOI email upon re-registering.

• Logged-in users do not require DOI confirmation for product subscriptions.
• They are directly notified when the product becomes available.