monero: check the node once a day, not every five minutes

A wedged explorer produced 83 Signal messages in six hours. Nothing about that
was a monitoring success: the checks were right, the cadence was wrong.

monerod-box is a Monero node in the middle of a multi-week initial sync on a
rotating disk. A 5-minute poll tells you nothing a daily one does not, and when
something breaks it turns one fault into a message every five minutes until
somebody silences the phone - which is the failure mode that makes people stop
reading alerts entirely.

So both Monero probes move to OnCalendar:

    monerod    *-*-* 05:00:00   RandomizedDelaySec=900
    xmrblocks  *-*-* 05:30:00   RandomizedDelaySec=900
    systemd units (infra/401 per-host timer)  *-*-* 05:15:00

30 minutes apart on purpose: both read the same LMDB, and the explorer's check
renders a full page out of it - a cold one measured 68 seconds. Running them
together would have each paying the other's cache misses.

OnCalendar rather than OnUnitActiveSec: an interval timer re-bases on every
restart, so "daily" silently becomes "a day after whenever it last came up".

── The window and the cadence are ONE decision ─────────────────────────────
The Gatus heartbeat moves with them, to 26h. Not 24h: the window must EXCEED
the push interval, or a check reporting exactly on schedule races its own
deadline and alerts on jitter alone. With a 900s randomized delay the true
worst case is ~24h15m, so 26h leaves real headroom.

The same logic drops the pulled public-site checks to a 24h interval - and
their failure-threshold from 3 to 1 with it. Three failures at 5m is 15 minutes
of tolerance; three at 24h would be THREE DAYS before anyone heard. A daily
check has no room for a blip, so it alerts on the first.

── Scoping ────────────────────────────────────────────────────────────────
infra/401's timer is ONE PER HOST and iterates every unit in
monitored_services, so slowing it slows all of them. That is safe here only
because monerod-box runs nothing but Monero units; the knobs are therefore in
its host_vars rather than the role, and both 401 and 403 now take a per-host
(resp. per-probe) override with the old 11m as the default. Every other host
is untouched.

Host liveness and disk are deliberately NOT slowed. They answer "is the box
alive", which is still wanted within minutes.

The role-level OnCalendar defaults are prefixed (monero_healthcheck_on_calendar,
xmrblocks_healthcheck_on_calendar) because the generic `healthcheck` role owns
the unprefixed name, and an unprefixed default here would shadow it for any
play that includes both.

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

ansible/roles/xmrblocks/defaults/main.yml

@@ -73,3 +73,9 @@ healthcheck_push_url: ""
 # Bearer token for the Gatus external endpoint. Required whenever a push URL
 # is set: Gatus rejects an unauthenticated push with 401.
 healthcheck_push_token: ""
+
+# WHEN TO CHECK. Daily, 30 minutes after monerod's own check so the two do not
+# contend for the same cold LMDB pages. See roles/monero/defaults for why the
+# name is prefixed and why this is paired with the 26h heartbeat in
+# infra/403_service_probe_registration.yml.
+xmrblocks_healthcheck_on_calendar: "*-*-* 05:30:00"

ansible/roles/xmrblocks/templates/healthcheck.timer.j2

@@ -7,7 +7,14 @@ Description=Onion Monero Blockchain Explorer Health Check Timer
 
 [Timer]
 OnBootSec=2min
-OnUnitActiveSec=5min
+# ONCE A DAY, not every 5 minutes. See the note in roles/monero's timer: the
+# explorer follows the node it reads from, and this check costs a full page
+# render out of the LMDB (a cold one measured 68 seconds), so polling it hard
+# is not free either. The Gatus heartbeat window is 26h, set in
+# infra/403_service_probe_registration.yml, and MUST stay above this interval.
+OnCalendar={{ xmrblocks_healthcheck_on_calendar }}
+# Spread against the other daily pushes leaving this box.
+RandomizedDelaySec=900
 Persistent=true
 
 [Install]