Feature request: Version history / rollback for Templates (campaign & transactional)

Is your feature request related to a problem? Please describe. Templates in listmonk (both campaign and transactional) can currently be overwritten with no way to recover the previous content. There is no undo, no history, and no confirmation step that warns you when you're about to replace the HTML of a template that is actively used by transactional emails. This isn't a hypothetical edge case. I build my templates locally (VSCode) and paste the finished HTML into the listmonk web UI. During one of these routine updates, I accidentally pasted the HTML into the wrong template and saved it. The previous HTML was gone instantly — there was no way to restore it from within listmonk. Because that template was wired to a transactional flow, the mistake wasn't just cosmetic: transactional emails started failing/rendering incorrectly right away, and end users were the ones who hit the errors. Recovering meant manually rebuilding the template from scratch, and every minute t…