💚 Pin Docker build to Ruby 3.4.7

The deploy got past setup-ruby but failed inside the image build with the same error: parallel 2.1.0 requires Ruby >= 3.3, but the build ran on 3.2.3. Two places still pinned the old version. config/deploy.yml is the decisive one, since kamal passes RUBY_VERSION as a build arg that overrides the Dockerfile default, so fixing the Dockerfile alone would not have changed the build. Both now match .ruby-version (3.4.7), as the comment above the ARG already asked for. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HCxJUTQkwwSCMoJpmuiZep