Rails CI on GitHub Actions

Asset precompilation and native gem builds dominate the pipeline.

Why it happens

vendor/bundle, node_modules and tmp/cache/assets are all cold on every run.

What to change

1. Cache vendor/bundle, node_modules and tmp/cache/assets together
2. Run RSpec with parallel_tests across all cores
3. Precompile assets only in the deploy job, not in the test job
4. Use a snapshot runner so bundle install disappears completely

Rails on runnerhut

jobs:
  test:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    services:
      postgres: { image: postgres:17, env: { POSTGRES_PASSWORD: postgres } }
    steps:
      - uses: actions/checkout@v4
      - uses: ruby/setup-ruby@v1
        with: { bundler-cache: true }
      - run: bundle exec rails db:test:prepare
      - run: bundle exec parallel_rspec spec/

The only runnerhut-specific line is `runs-on: runnerhut-8vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.