We took a super simple (IMO) approach to collaborative editing in my current project:

Each block of text has a version number which must be incremented by one by the client at the time of submission. The database provides conflict prevention by uniqueness constraint which bubbles up to the API code. The frontend is informed of conflict, so that the user can be notified and let the human being perform conflict resolution.

Because most concurrent users are working on different blocks, this works great.