Enable PKCE for ArgoCD OIDC login (#477)

The Authentik client for ArgoCD is now public (the iOS app can't hold
a secret), so Authentik no longer enforces client_secret on token
exchange. PKCE replaces that as the protection against
authorization-code interception.

- Add `enablePKCEAuthentication: true` to the `oidc.config` block in
  `argocd-cm-patch.yaml`
- Note why PKCE is needed now that the client is public

Reviewed-on: #477
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

clusters/au-syd1/bootstrap/argocd-cm-patch.yaml

@@ -26,6 +26,10 @@ data:
     issuer: https://identity.unkin.net/application/o/argocd/
     clientID: argocd
     clientSecret: $argocd-oidc:client_secret
+    # The Authentik client is public (the iOS app can't hold a secret), so
+    # Authentik no longer enforces clientSecret; PKCE replaces it as the
+    # protection against authorization-code interception.
+    enablePKCEAuthentication: true
     # identity.unkin.net now serves the LetsEncrypt *.unkin.net wildcard, so the
     # stock image trust store validates it; no rootCA pin.
     requestedScopes: