Fix race condition in Subscription#verify!

[bellisabell]

Problem

The verify! method in Subscription has a race condition that can send duplicate new subscriber notifications.

When two concurrent requests call verify! on the same subscription:

1. Both check verified_at.present? → both see nil
2. Both set verified_at and save
3. Both call send_new_subscriber_notification

Solution

Wrap the verify! method in with_lock to use pessimistic database row-level locking. This ensures only one process can read and modify the record at a time, making the check-then-act pattern atomic.

def verify!
  with_lock do
    already_verified = verified_at.present?
    self.verified_at = Time.zone.now
    self.unsubscribed_at = nil
    save!

    send_new_subscriber_notification unless already_verified
  end
end

Closes #29