fix(keycloak): readiness could not fail on the database, and the measured budget

Two corrections found while verifying the first CI run, not from the plan.

THE DEFAULT THAT MADE THE READINESS PROBE TOOTHLESS. Keycloak's DATABASE health
check is the one row of upstream's "Available Checks" table whose "Requires
Metrics" column says yes; Cluster, Graceful shutdown and Keycloak Initialized
say no. So with metrics-enabled=false - this chart's default until now -
/health/ready answers 200 while the database is gone, and the pod keeps taking
logins it cannot serve. The probe path was right and the aggregate behind it
contained nothing external, which is the urlaubsverwaltung defect (#218) in a
new hiding place: not a health group this time, but an option named for
observability, where nobody looks when reviewing a probe.

keycloak.metrics.enabled therefore defaults to TRUE, and its comment argues
about readiness rather than about scraping. The price is stated at the value:
/metrics then answers on the management port, unauthenticated, carrying realm
names, client ids and login counters - on a port that is ClusterIP-only and in
no Ingress. Switching it off is now documented as a decision about whether
readiness can fail at all. The CI fixture keeps it on and says why, so a green
run really does touch the database.

THE STARTUP BUDGET NOW CARRIES ITS MEASUREMENT. From the install-test of PR #87
on the Gitea docker runner (k3s v1.35.5, two replicas, empty database, image
pulled during the run): 71.8 seconds from `helm install` to BOTH pods Ready -
first at ~70.8s, second one second later, 0 restarts. The 600s budget stays,
with the reason for the headroom written down: everything in that run was the
cheap case, and a first install against a real database serializes the second
replica behind the first's Liquibase migration lock instead of letting it
arrive a second later.

AND AN OVERCLAIM OF MY OWN, REMOVED. ci/keycloak/test-values.yaml said a green
run proves jdbc-ping found the second replica. It does not: readiness reports
one node's own health, so a node that never found its peer is Ready, healthy
and alone - and two such nodes are two islands that each believe they are the
cluster. What the test does rule out is the louder half, a replica that cannot
start or cannot share the database. The Infinispan view has to be read out of
the log, which is what step 2 of NOTES.txt tells the operator to do, now with
the note that no probe and no install-test replaces that step. Measurement
point 4 of the plan therefore stays open.

Part of #83

keycloak/templates/_helpers.tpl

@@ -180,7 +180,7 @@ Usage: {{ $settings := fromYaml (include "keycloak.settings" .) }}
         "database" "keycloak"
         "pool" (dict "initialSize" 2 "minSize" 2 "maxSize" 20))
       "bootstrapAdmin" (dict "enabled" true)
-      "metrics" (dict "enabled" false)
+      "metrics" (dict "enabled" true)
       "features" (list)
       "cache" (dict "clusterName" nil)
       "ports" (dict "http" 8080 "management" 9000 "jgroups" 7800)
@@ -392,11 +392,25 @@ away, which is also why the wrong path would look entirely plausible in a diff.
 {{- end -}}
 
 {{/*
-READINESS: /health/ready - which DOES include the database, and must. A node
-that cannot reach its database answers every login with an error, so no Service
+READINESS: /health/ready - which includes the database, and must. A node that
+cannot reach its database answers every login with an error, so no Service
 should route to it. This is the other half of guideline 12.3: a dependency-free
 liveness path is only correct when readiness really does hang on the
 dependency.
+
+AND THE PATH ALONE DOES NOT GUARANTEE THAT, which is the part worth reading
+before anybody "cleans up" keycloak.metrics.enabled. Upstream's table of
+available checks carries a "Requires Metrics" column, and the DATABASE check -
+"Returns the status of the database connection pool" - is the row that says
+YES; Cluster, Graceful shutdown and Keycloak Initialized say no. So with
+metrics-enabled=false this endpoint still answers 200 while the database is
+gone, and the pod keeps taking logins it cannot serve.
+
+That is exactly the urlaubsverwaltung shape (#218): the probe path was right
+and the aggregate behind it contained nothing external. Here the dependency
+hides behind an option whose name says observability, which is why
+keycloak.metrics.enabled defaults to TRUE and why its comment in values.yaml
+argues about readiness rather than about scraping.
 */}}
 {{- define "keycloak.readinessProbe" -}}
 {{- $defaults := dict