monitoring: recover host checks for the whole estate, reported to Gatus

Five checks, 27 endpoints, replacing what Uptime Kuma used to watch:

  is it up      every 5min, all hosts
  is disk full  daily,      all hosts
  is CPU hot    every 5min, nodito
  is ZFS broken daily,      nodito
  is UPS online every 5min, nodito

Two roles, kept separate so neither knows about the other - they meet at a URL
and a token, the same way caddy_site and each service meet at a vhost:

  roles/gatus_endpoint  runs on the observability host, writes ONE file into
                        /opt/gatus/config/endpoints/. Gatus merges every *.yaml
                        there and appends lists, so callers compose without
                        coordinating.
  roles/healthcheck     runs on the monitored host: a check script, a systemd
                        service, a timer, and an optional push. Ships a library
                        of check bodies under templates/checks/.

Everything PUSHES. Gatus never reaches out, which matters because nodito and its
VMs are behind NAT, and because four of the five checks are internal state with
no pollable surface at all. Liveness pushes too, deliberately: a heartbeat proves
the host is running AND can reach the internet, where an ICMP probe from one
vantage point only proves it answers pings from there. And since Gatus alerts
when a heartbeat window expires, a check that stops running raises the alarm by
itself - a dead timer looks exactly like a dead host, which is the correct
reading.

One bearer token per host, generated straight into the vault and never printed.
A token only writes results for its own host's endpoints, so a compromised host
can lie about itself, which it could do anyway.

Three things learned from the source that shaped this:

  * Gatus polls its own config every 30s and reloads
    (main.listenToConfigurationFileChanges), so gatus_endpoint needs no restart
    handler - writing the file IS the deploy.
  * ...but on a reload it panics if the new config fails to parse, unless
    skip-invalid-config-update is set. Endpoint files are contributed by other
    playbooks, so one malformed file would take the monitor down at the worst
    possible moment. Now set.
  * The push URL uses a key Gatus computes, not the name you write:
    sanitize(group) + "_" + sanitize(name), lowercased with / _ . , space # + &
    replaced by "-" (config/key/key.go). So knots_box_local is
    knots-box-local in the URL. The playbook derives it rather than hand-writing.

storage: maximum-number-of-results 900, up from upstream's 100. Gatus bounds the
database by COUNT and trims inline on insert, so there is no retention job and
no way to fill a disk - but history depth is then a function of check frequency,
and 100 results at a 5-minute interval is 8 hours. 900 is ~3 days of liveness
and ~2.5 years of the daily disk check. The uptime table is separate and its
30-day retention is hard-coded upstream.

A bug worth recording: the first deploy shipped five scripts that all died with
"syntax error: unexpected end of file". Jinja strips an included template's
trailing newline and trim_blocks then eats the newline after {% endif %}, so the
closing brace of check() landed on the same line as the body's last statement -
`return 0}`. Every check was broken and the deploy still reported failed=0,
because the role's "run once" task has failed_when: false and reports the result
as a debug message nobody read. The blank line that fixes it is now load-bearing
and commented as such.

Verified by triggering every unit by hand rather than waiting on timers: all
checks exit 0 on all hosts, and Gatus shows 26 UP / 1 DOWN. The one DOWN is
liveness_watchtower, which is honest - that host currently refuses SSH (TCP
connects, no banner exchange) and is excluded from this deploy. It is also the
box still running Uptime Kuma.

Known waste, not yet fixed: healthcheck installs its dependencies per CHECK
rather than per HOST, so apt runs 29 times estate-wide for a curl that is
already present, and daemon_reload runs 4x per host.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ansible/roles/gatus_endpoint/defaults/main.yml

@@ -0,0 +1,28 @@
+---
+# One invocation writes ONE file into Gatus's endpoints directory. Gatus merges
+# every *.yaml under GATUS_CONFIG_PATH and appends lists, so each caller owns
+# its own file and they compose without coordinating - the same shape as
+# caddy_site, where each service contributes its own vhost.
+
+# Filename stem: <name>.yaml
+gatus_endpoint_name: ""
+
+# PULLED endpoints - Gatus makes the request and evaluates conditions.
+#   - {name, group, url, interval, conditions: [...], alerts: [...]}
+gatus_endpoint_pulled: []
+
+# EXTERNAL endpoints - the host pushes its own result. Gatus never reaches out,
+# which is what makes this work for machines behind NAT and for state that has
+# no pollable surface at all (disk usage, ZFS health, UPS mains).
+#
+#   - {name, group, token, heartbeat, alerts: [...]}
+#
+# `heartbeat` is the important one: if nothing reports within that window Gatus
+# alerts. That is what makes a push check detect its own failure - a dead timer
+# looks exactly like a dead host, which is the correct reading.
+gatus_endpoint_external: []
+
+# Where the files live. Matches roles/gatus.
+gatus_config_dir: /opt/gatus/config
+gatus_endpoints_dir: "{{ gatus_config_dir }}/endpoints"
+gatus_gid: 10001