Put the artifactapi web UI behind Authentik oauth2-proxy

Front /ui with an Authentik-authenticated oauth2-proxy while leaving every
package-manager surface unauthenticated.

- Add the oauth2-proxy ConfigMap, Deployment, Service and VMPodScrape.
- Add the oauth-credentials VaultStaticSecret.
- Split httproute: /ui and /oauth2 to oauth2-proxy (HTTPS, plus an HTTP
  redirect); everything else to the api Service on both listeners.

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