Add additional mechanisms to prevent deletion of overage_terms_acceptances records

The following discussion from !13864 should be addressed:

• @paulobarros started a discussion: (+1 comment)

  This won't protect against raw DELETE queries, for example. Also calling delete_all could bypass this, I think.

  Maybe it's better to enforce this on the database-level with a trigger? Or at least prevent the most common ways to bypass at the Rails level? 🤔

  Happy to involve a database maintainer here.