v0.3.0 · Prebuilt engine updates

Highlights

• The engine can now install a prebuilt release: rerun update --artifact <url> --sha256 <hex> --commit <sha> downloads it, checks it, migrates the data directory with the new build, switches current to it and restarts, keeping the previous release beside it. rerun rollback swaps back in a second, and the systemd unit rolls back once on its own if the new build fails to start.
• The engine knows how it is installed (release, git monorepo, or Docker image) and /health reports it, along with the status of the last update.
• Sign-in page: a "Last used" badge marks the method you signed in with last time.

Fixes

• An update that failed (checksum, disk, migration) no longer leaves the engine answering "already updating" until its next restart.
• The browser installer no longer needs pnpm in a release or an image.

Upgrade

• Nothing to do: pull the new images. rerun update inside the image now says so instead of failing.