Version history is a feature you didn't build

A customer emails: they overwrote Tuesday's version of their project file and the board meeting is Thursday. Your roadmap has "document history" scheduled for next quarter.

Except each customer lives in their own container, and the files service journals every mutation on that container's disk. Your app calls ?history on their file, lists the revisions, fetches ?revision=N, shows a diff with ?diff. You build the UI in a day; the versioning itself has been running since the customer signed up.

Competitors architected snapshot tables, delta storage, garbage collection. You expose an endpoint the substrate was already serving.