Reported by @1000Delta. The printer file download (and three sibling
endpoints) raised UnicodeEncodeError: 'latin-1' codec can't encode
characters... on any filename outside U+0000..U+00FF (Chinese,
Japanese, Arabic, accented Latin), because the route pushed `filename`
straight into Content-Disposition: attachment; filename="...".
Starlette/uvicorn encodes response headers as latin-1, so the assignment
crashed at write-time.
New backend/app/utils/http.py::build_content_disposition emits both an
ASCII-stripped legacy filename="..." fallback and an RFC 5987
filename*=UTF-8''<percent-encoded> parameter. Every modern browser
prefers the *= form, so the original Unicode filename round-trips
through Save-As intact.
Same shape was latent in three siblings and fixed in the same PR
(no deferred follow-ups): archive QR endpoint (archive.print_name
from 3MF metadata), project ZIP export (project.name — the existing
isalnum() sanitiser passes non-ASCII through), and the PDF label
streamer (latent today, callers ASCII-only but the helper hardens it).
