Put the artifactapi web UI behind Authentik oauth2-proxy (#456)

The artifactapi web UI is open to anyone who can reach the host. Front it with Authentik SSO gated on akP-artifactapi-admin, while leaving the package-manager surfaces (/api/v1, /api/v2, /v2 docker registry, /terraform, /.well-known) untouched — dnf, containerd mirrors, buildah, terraform and CI publish steps cannot do a browser flow.

- Add the oauth2-proxy ConfigMap, Deployment, Service and VMPodScrape.
- Add the oauth-credentials VaultStaticSecret.
- Point the api-route /ui rule at oauth2-proxy and add a /oauth2 rule; the catch-all / rule still goes straight to the api Service on both listeners.

Requires terraform-authentik #34 applied and the Vault kv seed first.

Reviewed-on: #456
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/artifactapi/vmpodscrape.yaml

@@ -14,3 +14,17 @@ spec:
   podMetricsEndpoints:
     - port: metrics
       path: /metrics
+---
+# Scrape the UI oauth2-proxy (:44180), which exposes sign-in/authz counters.
+apiVersion: operator.victoriametrics.com/v1beta1
+kind: VMPodScrape
+metadata:
+  name: oauth2
+  namespace: artifactapi
+spec:
+  selector:
+    matchLabels:
+      app: oauth2
+  podMetricsEndpoints:
+    - port: metrics
+      path: /metrics