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>

README.md

@@ -2,6 +2,8 @@
 
 > *Linderhof* — the smallest and most intimate of Ludwig II's Bavarian palaces, the only one he lived to see completed; built entirely to his own vision as a private retreat. ([Wikipedia](https://en.wikipedia.org/wiki/Linderhof_Palace))
 
+**[codeberg.org/opennomad/linderhof](https://codeberg.org/opennomad/linderhof)**
+
 a self-hosting stack based on ansible and docker compose that comes with
 
 - email