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/infra/402_public_monitoring.yml

@@ -21,7 +21,7 @@
     # then left behind when the machine moved, so the check asserts against
     # ansible_host - if a box is renumbered, inventory is the one edit.
     dns_records:
-      - {sub: "{{ subdomains.gatus }}",              host: monitoring}
+      - {sub: "{{ subdomains.gatus }}",              host: spacey}
       - {sub: "{{ subdomains.headscale }}",          host: spacey}
       - {sub: "{{ subdomains.vaultwarden }}",        host: vipy}
       - {sub: "{{ subdomains.forgejo }}",            host: vipy}