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

This reverts commit c98d88c197.

The artifactapi web UI has been down since #456 merged: /ui/ and /oauth2/
both return 503 "no available server", Traefik's response for a Service
with no ready endpoints, so the oauth2-proxy pod is not becoming ready.
The machine API surface (/version, /api/v2/health, /v2/,
/.well-known/terraform.json) is unaffected and still returns 200.

Rolling back restores unauthenticated access to the UI at /ui, served
directly by the ui Service exactly as before.

- Point the api-route /ui rule back at the ui Service and drop the
  /oauth2 rule.
- Remove the oauth2-proxy ConfigMap, Deployment, Service and VMPodScrape.
- Remove the oauth-credentials VaultStaticSecret.

The apps/base/artifactapi tree is byte-identical to 520da44, the commit
immediately before #456. Nothing that landed since is touched.

apps/base/artifactapi/services.yaml

@@ -16,26 +16,6 @@ spec:
   sessionAffinity: None
   type: ClusterIP
 ---
-# Authenticated front door for the web UI only: api-route sends /ui and /oauth2
-# here, oauth2-proxy authenticates and forwards to the ui Service. Every other
-# path reaches the api Service above directly and stays unauthenticated.
-apiVersion: v1
-kind: Service
-metadata:
-  name: oauth2
-  namespace: artifactapi
-spec:
-  internalTrafficPolicy: Cluster
-  ports:
-    - name: http
-      port: 80
-      protocol: TCP
-      targetPort: http
-  selector:
-    app: oauth2
-  sessionAffinity: None
-  type: ClusterIP
----
 apiVersion: v1
 kind: Service
 metadata: