rebuild: fold monitoring into spacey and prd-arbret into vipy

All four droplets were deleted by mistake on 2026-10-17 and rebuilt as two:
Gatus and signal-api now share spacey with headscale, and arbret moves onto
vipy. The old IPs were recycled to strangers within a day.

- inventory: [observability] is spacey. Left as it was, the bare name
  `monitoring` resolved through ~/.ssh/config to an IP that now belongs to
  someone else. [arbret]/prd-arbret is gone.
- inventory: lan_ip fallbacks corrected from the Proxmox guest agent -
  memos-box is .130 (was .145) and arbret-staging-box .145 (was .147).
- spacey: gatus_port 8081. headscale already holds 0.0.0.0:8080, and the two
  never shared a host before.
- 402: status.<domain> is expected to resolve to spacey.
- headscale ACL: prd-arbret, prd-monitoring and stalwart-box removed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ansible/host_vars/spacey/main.yml

@@ -11,3 +11,10 @@
 monitored_services:
   - headscale
   - caddy
+
+# Gatus moved onto this host in the 2026-10-17 rebuild (the `monitoring` droplet
+# is gone). Headscale already listens on 0.0.0.0:8080, so Gatus takes 8081.
+# Host vars rather than role defaults: the Caddy vhost play in
+# services/gatus/deploy_gatus_playbook.yml does not load the role and reads
+# this same variable.
+gatus_port: 8081