Cairn v0.1.0-alpha.2

First public pre-release. Same server as 0.1.0-alpha.1; this release adds easier ways to start it.
Binaries and the image are built on GitHub-hosted runners and download without a GitHub account.

Added

• Public multi-arch image ghcr.io/solutionforest/cairn (linux/amd64, linux/arm64) and a
  root compose.yaml: docker compose up -d, with --profile ui for the admin UI.
• scripts/start.sh: one command downloads and verifies the release binary, creates a data
  directory with fresh credentials, and starts the server (and with --ui, the admin UI).
• Optional local admin UI (admin-ui/): status, buckets, users, keys, and one-click restore from
  the trash. Runs with Python 3 or PHP and needs no packages.