feat(prod): observability for the two failure modes that went unseen, plus OIDC self-healing

Minimal prod delivery, authored directly on main (= prod's live revision d1a6628)
rather than by merging work/prod-standup-campaign. That branch carries the DNS apex
inversion, which would have set prod's BASE_DOMAIN to home.keithjackson.dev; prod
runs external-dns with policy: sync and txtOwnerId: prod, so it would have begun
rewriting apex records dev owns while the non-k3s prod still serves 443 there.
Operator sequencing is "dev migrates first". Nothing under clusters/ is touched
here at all, which is what makes this safe to land now.

Also deliberately EXCLUDED, because prod's live cluster-vars has no CNPG_BACKUP_PATH
key and StrictPostBuildSubstitutions would fail the whole tier: the R2 rework of
platform/velero/ and the CNPG destinationPath changes. Those land as
feat/r2-backup-prod together with the endpoint and bucket knobs.

WHAT THIS FIXES

1. Nothing was watching CNPG. All three prod databases have run since bootstrap with
   ContinuousArchiving=False and firstRecoverabilityPoint=null, and no alert existed
   anywhere. Because archive_mode=on and the archive command fails, Postgres refuses
   to recycle WAL: keycloak-db holds every segment ever written (257 MiB on a 5 GiB
   volume, ~6.8 MiB/h) — roughly 29 days to a disk-full PANIC on the cluster's SSO
   database. Found by hand, 38h late. CNPGWALArchiveBacklog would have fired within
   the hour, and evaluates to 3 firing series on prod today.
   Rules key on cnpg_collector_* rather than cnpg_pg_stat_archiver_*, because
   nextcloud-db's exporter cannot log in to its own database (permission denied for
   database "nextcloud", both clusters) so the pg_stat_archiver family does not exist
   for it — the obvious metric would have been blind to one database in three.

2. No PVC outside namespace=monitoring had any fill alert. New rules are partitioned
   by namespace so they cannot double-fire with MonitoringVolumeFillingUp, pinned to
   job="kubelet" (the apiserver copy of the same series reports namespace=default),
   and guarded by inodes > 0 — block-mode PVCs such as unifi-rootdisk report
   available=0 forever and would otherwise have fired critical permanently.

3. VPNTunnelDown could never clear on a cluster with no gateway node; it has been
   firing critical on prod since 2026-09-05. Now guarded so prod self-arms the moment
   a node carries the egress-gateway label, with dev's behaviour unchanged today.

4. The Keycloak realm import fails OPEN: an unresolved placeholder is stored
   LITERALLY and the import still reports success, then sits at Done=True and never
   re-runs. On 2026-09-06 all 12 prod OIDC clients held the literal
   $(env:..._CLIENT_SECRET) string and every SSO login failed, while every signal was
   green. The CronJob is the level-triggered correction; two alerts make its silence
   observable. Repaired live already — this stops it recurring.

Also: the R-NET-04 conformance scan descended into nested agent worktrees under
.claude/, counting 4 egress policies where 1 is authored.

Co-Authored-By: Claude Opus 5 <[email protected]>

platform/monitoring/kube-prometheus-stack.yaml

@@ -221,6 +221,17 @@ spec:
         limits:
           memory: 256Mi
     kube-state-metrics:
+      # Expose the egress-gateway NODE LABEL as a metric label so alerts can ask "does this cluster
+      # have a VPN egress gateway at all?". KSM exposes NO custom labels by default, so without this
+      # `kube_node_labels` does not exist and any join against it silently matches nothing.
+      # Consumed by `VPNTunnelDown` (platform/monitoring/alerts.yaml): the alert is armed only on a
+      # cluster that actually has a labelled gateway node, and ARMS ITSELF the moment one is labelled
+      # — which is how prod (no gateway today) stops emailing a permanently-firing critical alert
+      # without anyone having to remember to re-enable it later. Scoped to one label on one resource:
+      # 3 extra series, no cardinality concern. Label name is sanitized by KSM to
+      # `label_homelab_keithjackson_dev_egress_gateway` (`label_` + non-alphanumerics -> `_`).
+      metricLabelsAllowlist:
+        - "nodes=[homelab.keithjackson.dev/egress-gateway]"
       resources:
         requests:
           cpu: 20m