ops(ansible): harden provisioning and restore automation

Fail before host changes when required deployment variables are missing.

Precreate the PostgreSQL temporary directory and escape Bash syntax during Jinja rendering.

ansible/roles/wal_g/templates/db-sync.sh.j2

@@ -17,7 +17,9 @@ systemctl stop "$POSTGRES_SERVICE"
 
 shopt -s dotglob nullglob
 contents=("$PGDATA"/*)
+{% raw %}
 if (( ${#contents[@]} > 0 )); then
+{% endraw %}
   rm -rf --one-file-system -- "${contents[@]}"
 fi