CaseMark stops hiding the demo paywall behind 404s

Document previews in CaseMark's fork were failing with generic not-found errors, even though the files existed and were ready to serve. The actual cause was a demo budget cap firing inside Case Vault, the storage layer - but that signal was getting flattened into nulls on the way out, so users and operators alike saw the wrong story.

kveton rewired the error path so the budget signal survives all the way from storage through to the viewer. The DOCX preview is now streamed through CaseMark's own backend rather than redirecting the browser off to a signed storage URL, which keeps the error inside one trusted hop where it can be read correctly. PDF and DOCX viewers now show a plain demo-budget message instead of a cryptic load failure. From open to merge: about twenty-five minutes.