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/services/headscale/templates/acl.json.j2

@@ -46,12 +46,7 @@
         "tag:jar-lid-archive-staging-box:8000",
         "tag:monerod-box:18089",
         "tag:monerod-box:8081",
-        "tag:arbret-staging-box:80",
-        "tag:stalwart-box:25",
-        "tag:stalwart-box:465",
-        "tag:stalwart-box:993",
-        "tag:stalwart-box:4190",
-        "tag:stalwart-box:8080"
+        "tag:arbret-staging-box:80"
       ],
       "src": [
         "tag:prd-vipy"
@@ -80,10 +75,8 @@
     {
       "action": "accept",
       "dst": [
-        "tag:prd-arbret:22",
         "tag:prd-vipy:22",
         "tag:memos-box:22",
-        "tag:stalwart-box:22",
         "tag:jar-lid-archive-staging-box:22"
       ],
       "src": [
@@ -125,20 +118,11 @@
     "tag:nodito": [
       "{{ headscale_namespace }}@"
     ],
-    "tag:prd-arbret": [
-      "{{ headscale_namespace }}@"
-    ],
-    "tag:prd-monitoring": [
-      "{{ headscale_namespace }}@"
-    ],
     "tag:prd-vipy": [
       "{{ headscale_namespace }}@"
     ],
     "tag:small-backups-box": [
       "{{ headscale_namespace }}@"
-    ],
-    "tag:stalwart-box": [
-      "{{ headscale_namespace }}@"
     ]
   }
 }