💚 Build with Ruby 3.4.9 and clear the Brakeman warnings

config/deploy.yml passed RUBY_VERSION=3.2.3 to the Docker build, which overrode the Dockerfile and failed on rbs needing Ruby 3.3+. The backup size now comes from File.size instead of a shell call, and the weak link_to finding is recorded in config/brakeman.ignore, so scan_ruby exits 0.