Every mutating form now carries the CSRF token as a hidden field, and the
upload route accepts it as a multipart field as well as a header. Driving the
real UI in Firefox found what the header-only tests could not: Create Draft,
Save, Publish, Delete and the up/down buttons all posted without a token and
were rejected. Adds npm run check-site, which verifies a running Site from the
outside and exits non-zero so it can gate a cutover.
