This question is in line with What is an effective way to copyright my teaching material? but about preprint work, not course material.
I produced a huge original database during a COVID break (sans any financial support) on my PhD which will not be used in the thesis, since the subject changed when I returned. It is about to be published as a book, but work on this is slow.
A prof from another department is asking to see the material to use in a class on that subject. Which I may participate but I'm not counting on this. I have no reason to distrust this other professor, but at the same time my colleagues keep reminding me of previous cases of stolen research and topics and materials that happened on that department and are widely known (from other professors, but recent, verifiable, and largely unpunished).
Just to be safe, what are some ways I can protect my work? I have under a month for preparations on this. I do not mind about my material being used as coursework, or being credited (but would be nice if it is possible to enforce somehow). I do wish to protect from competing publications.
I'm thinking about just slapping a copyright right notice at the footer and keeping a recording of the first online call where we will talk about the material/database.