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/caddy/tasks/main.yml

@@ -38,6 +38,16 @@
     mode: "0775" # also allow members of the docker group to write
   loop: "{{ caddy_sites }}"
 
+- name: Deploy default landing page for empty sites
+  template:
+    src: index.html.j2
+    dest: "/srv/caddy/sites/{{ item }}/index.html"
+    owner: root
+    group: docker
+    mode: "0644"
+    force: false  # never overwrite real content
+  loop: "{{ caddy_sites }}"
+
 - name: Install Caddyfile
   template:
     src: Caddyfile.j2