Add landing page, Hetzner labels, and Codeberg link

- Add default landing page (roles/caddy/templates/index.html.j2) deployed
  to empty caddy sites; adapted from YC/coming-soon by Steven Tang (MIT),
  with site domain and powered-by footer linking to codeberg.org/opennomad/linderhof
- Apply hcloud_labels to all Hetzner cloud and DNS resources; default to {}
  in role defaults for stacks without the variable defined
- Fix setup.sh: export stack_name so envsubst substitutes it in config.yml
- Add Codeberg repo link to README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

roles/provision/defaults/main.yml

@@ -3,3 +3,4 @@ cloud_provider: hetzner
 hcloud_server_type: cx23
 hcloud_image: ubuntu-24.04
 hcloud_location: fsn1
+hcloud_labels: {}  # override in config.yml — see config.yml.setup for recommended labels