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]>

infra/ansible/inventory/hosts.ini

@@ -31,10 +31,17 @@ k3s_secondary
 [hypervisor]
 localhost ansible_connection=local
 
-# VPN egress-gateway node(s) — run the node-level split-tunnel OpenVPN + kill-switch
-# (ADR-0015, playbooks/vpn.egress.gateway.yml). Initially just k3s-dev-3 (single gateway;
-# add a second host here for Cilium Egress Gateway HA). Members are already defined above
-# with their ansible_host under k3s_secondary; this group only tags the egress role target.
+# VPN egress-gateway node — runs the node-level split-tunnel OpenVPN + kill-switch
+# (ADR-0015, playbooks/vpn.egress.gateway.yml). EXACTLY ONE host belongs here.
+#
+# THIS GROUP IS SINGLETON BY DESIGN — a second host does NOT buy failover. Cilium OSS has no
+# egress-gateway failover: the CRD's own `egressGateway` description says "In case multiple nodes
+# are a match for the given set of labels, the first node in lexical ordering based on their name
+# will be selected". Two labelled nodes therefore means one silently wins by NAME ORDER (a labelled
+# `k3s-prod-1` beats a labelled `k3s-prod-3`) and the other carries no traffic. See
+# platform/vpn-egress/README.md "Resilience — there is no gateway failover" for the full statement
+# and the ADR-0016 escalation. Members are already defined above with their ansible_host under
+# k3s_secondary; this group only tags the egress role target.
 [vpn_egress_gateway]
 k3s-dev-3