fix(stacks): show correct status when backup service has failed

Same fix as the update service: gate "not installed" on unit file
existence rather than systemctl exit code, which is non-zero for
failed services too.

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

stacks.sh

@@ -664,8 +664,11 @@ EOF
       ;;
 
     status)
-      systemctl status stacks-backup.timer stacks-backup.service --no-pager 2>/dev/null \
-        || warn "Backup service not installed"
+      if systemctl cat stacks-backup.timer &>/dev/null; then
+        systemctl status stacks-backup.timer stacks-backup.service --no-pager 2>/dev/null || true
+      else
+        warn "Backup service not installed"
+      fi
       ;;
 
     logs)