Delivery responses carry no ETag or Last-Modified, so nothing can answer 304

Public delivery sets a 60 second `max-age` and nothing else. After that window every revalidation is a full body transfer, because there is no validator for a client or a CDN to send back.

`Content.UpdatedAt` is already stored and is enough to build both a `Last-Modified` and a weak `ETag`.

Worth doing once the cache key question in the delivery `Vary` issue is settled, since both touch the same responses.

Found Sept 5 2026 while working through read load.