On arm64, cpanm does not automatically resolve Mail::DKIM as a
transitive dependency of Mail::Milter::Authentication, causing the
build to fail. Install it explicitly before Mail::Milter::Authentication.
Embed rspamd-symbols.json in the binary to provide human-readable
descriptions for rspamd symbols in reports. Optionally fetch fresh
symbols from a configurable rspamd API URL (--rspamd-api-url flag),
falling back to the embedded list on error. Update the frontend to
display descriptions alongside symbol names and scores.
