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/base/kustomization.yaml

@@ -17,4 +17,8 @@ resources:
   - apps-wave-2.yaml     # Flux Kustomization -> ./apps/wave-2 (dependsOn apps-wave-1)
   # VM tier (KubeVirt) — non-gating (wait:false), separate lifecycle from the app waves.
   - apps-vms.yaml        # Flux Kustomization -> ./apps/vms (dependsOn platform-config)
+  # oauth2-proxy forward-auth — POST-platform tier (dependsOn platform-config, wait:false). A leaf
+  # service that needs Keycloak + trusted TLS + split-horizon DNS must NOT sit in the gating
+  # `platform` tier, where it self-gated its own dependencies into a cold-start deadlock (twice).
+  - oauth2-proxy.yaml    # Flux Kustomization -> ./platform/oauth2-proxy (dependsOn platform-config)
   - cluster-vars.yaml    # overlay knob surface (base defaults)