Demo worker

The demo instance (compose/docker-compose.demo.yaml) pairs the server with a dummy worker that completes every job with synthetic near-empty packages after DEMO_BUILD_SECS seconds — no compilation runs anywhere. It exists so visitors can add AUR packages and watch dependency resolution and publishing happen in seconds.

Deliberately the only published port is the web UI (8080): the pacman repository is not exposed, because the demo's packages are synthetic and must never be installed, and the worker protocol stays inside the compose network.

The dummy worker is I/O-free, so it runs unprivileged with a pinned WORKER_CONCURRENCY=4 to drain the demo queue quickly. Like a real worker it enrolls through the shared enroll volume and keeps its identity in its data volume.