logging: deploy logviewer v0.1.0 behind oauth2-proxy (#404)

## Why

The logviewer UI gives browser access to the ClickHouse log store (`logreader` user). It must only be reachable by authorized users, so it sits behind an Authentik-gated oauth2-proxy front at https://logviewer.unkin.net, mirroring the arrstack front-door and mediamover/pdbmux gateway patterns.

## Changes

- Adds the logviewer Deployment (1 replica, `docker-internal/logviewer:v0.1.0`) reading ClickHouse at `clickhouse-logs` via the existing `clickhouse-logreader` Secret, `/healthz` probes, nonroot securityContext
- Adds an oauth2-proxy (quay.io/oauth2-proxy/oauth2-proxy:v7.15.3) front with issuer `https://identity.unkin.net/application/o/logviewer/`, `ak_groups` claim, allowed group `akP-logviewer-admin`, internal-CA trust via the combine-certs initContainer
- Syncs kv `kubernetes/namespace/logging/default/oauth-credentials` into the `logviewer-oauth-credentials` Secret via the existing `default` VaultAuth (kv path already seeded; no terraform-vault change needed)
- Adds ClusterIP Services for the app and the oauth2-proxy front door
- Adds internal Gateway + HTTPRoutes for `logviewer.unkin.net` (vault-issuer cert, external-dns -> 198.18.200.4, http->https redirect)
- Registers `logviewer/` in the logging base kustomization

## Dependency

Login requires the Authentik `logviewer` provider/application from terraform-authentik PR #21. This PR is safe to merge first: the VSO secret sync and the deployment come up regardless; OIDC login only works once #21 is applied.

Validated with `kustomize build` (base + au-syd1 overlay) and kubeconform (49/49 valid).

Reviewed-on: #404
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/logging/logviewer/service.yaml

@@ -0,0 +1,36 @@
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: logviewer
+  namespace: logging
+spec:
+  internalTrafficPolicy: Cluster
+  ports:
+    - name: http
+      port: 8080
+      protocol: TCP
+      targetPort: http
+  selector:
+    app: logviewer
+  sessionAffinity: None
+  type: ClusterIP
+---
+# Front-door entry Service: the HTTPRoute for logviewer.unkin.net targets this;
+# all traffic enters via oauth2-proxy.
+apiVersion: v1
+kind: Service
+metadata:
+  name: logviewer-oauth2
+  namespace: logging
+spec:
+  internalTrafficPolicy: Cluster
+  ports:
+    - name: http
+      port: 80
+      protocol: TCP
+      targetPort: http
+  selector:
+    app: logviewer-oauth2
+  sessionAffinity: None
+  type: ClusterIP