fix(flux): move oauth2-proxy to a post-platform tier (dependsOn platform-config)

oauth2-proxy is a leaf forward-auth service that depends on Keycloak +
trusted wildcard TLS + split-horizon DNS. It lived in the gating `platform`
Flux Kustomization (wait:true), so on a cold start it Failed until those
dependencies existed, keeping the whole `platform` tier not-Ready. That in
turn blocked `platform-config` — which brings up the very Keycloak instance,
wildcard Certificate and L2 VIP oauth2-proxy needs (and previously carried
oauth2-proxy's own httproute/servicemonitor) — from ever reconciling. A
self-gating cold-start deadlock, twice-proven (DECISIONS-LOG sessions 7 & 9).

Break the cycle by moving the whole oauth2-proxy component out of the
`platform` tier into its own non-gating post-platform Flux Kustomization:

- clusters/base/oauth2-proxy.yaml: new Kustomization -> ./platform/oauth2-proxy,
  dependsOn platform-config, wait:false, substituteFrom cluster-vars.
- clusters/base/kustomization.yaml: reference the new Kustomization.
- platform/kustomization.yaml: drop the oauth2-proxy component (no longer in
  the gating tier).
- platform/oauth2-proxy/kustomization.yaml: pull in httproute + servicemonitor
  so the full component travels together.
- platform-config/kustomization.yaml: drop the in-place oauth2-proxy
  httproute/servicemonitor refs (moved with the controller).
- clusters/dev/kustomization.yaml: add the SOPS decryption patch for the new
  oauth2-proxy Kustomization (it now owns the encrypted credentials Secret that
  the `platform` Kustomization used to decrypt).

Now `platform` reaches Ready without oauth2-proxy; platform-config brings up
Keycloak/TLS/DNS; then oauth2-proxy reconciles against dependencies that
already exist. wait:false guarantees its health can never gate anything above
it. No config of oauth2-proxy itself changed — only its reconciliation tier.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

clusters/dev/kustomization.yaml

@@ -63,6 +63,20 @@ patches:
           provider: sops
           secretRef:
             name: sops-age
+  # oauth2-proxy moved to its own post-platform Flux Kustomization (clusters/base/oauth2-proxy.yaml).
+  # It carries oauth2-proxy-credentials.sops.yaml, previously decrypted via the `platform`
+  # Kustomization; now that this tier owns the encrypted Secret it needs its own decryption stanza.
+  - patch: |-
+      apiVersion: kustomize.toolkit.fluxcd.io/v1
+      kind: Kustomization
+      metadata:
+        name: oauth2-proxy
+        namespace: flux-system
+      spec:
+        decryption:
+          provider: sops
+          secretRef:
+            name: sops-age
   - patch: |-
       apiVersion: source.toolkit.fluxcd.io/v1
       kind: GitRepository