Fix provisioning errors found during first real run

- ansible.cfg: suppress Python interpreter discovery warning
- ansible/inventory.ini: add ansible_host; server IP is now defined in
  one place and read dynamically by run-bootstrap.sh
- group_vars/all/ → directory layout so vault.yml is auto-loaded by Ansible
  (previously vault.yml did not match any group name and was silently ignored)
- scripts/run-bootstrap.sh: read server IP from inventory, chmod 600 private
  keys automatically, show actual SSH error when root login fails
- scripts/bootstrap-deploy-user.sh: add sudoers.d entry for passwordless sudo
  (deploy user has no password so sudo group membership alone was not enough)
- nextcloud: fix Redis healthcheck (CMD-SHELL pipe was unreliable in Alpine,
  switched to CMD form with start_period)
- group_vars/all/vars.yml: fix Roundcube image tag (1.6-apache and 1.6 do not
  exist; correct tag is 1.6.x-apache)
- docs/runbook.md: expand prerequisites (SSH keys section), add step 1 for
  setting the server IP, expand bootstrap step with preflight detail, fix
  step numbering

scripts/check-vault.sh

@@ -7,7 +7,7 @@ set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 REPO_ROOT="$(cd "${SCRIPT_DIR}/.." && pwd)"
-VAULT_FILE="${REPO_ROOT}/ansible/group_vars/vault.yml"
+VAULT_FILE="${REPO_ROOT}/ansible/group_vars/all/vault.yml"
 
 REQUIRED_KEYS=(
   traefik_dashboard_users