platform: move Headlamp out of the gating platform tier + fix -session-ttl crashloop

Headlamp's HelmRelease sat in the wait:true `platform` Flux tier. Its pod
crashlooped (`flag provided but not defined: -session-ttl` — chart 0.40.1
renders a container arg the appVersion v0.40.1 backend lacks; upstream
kubernetes-sigs/headlamp#4883), and because it gated `platform`, the whole
chain stalled: platform Ready=Unknown -> platform-config, oauth2-proxy,
apps-wave-0/1/2 and apps-vms all Ready=False "dependency not ready".

Part A (unblock + durable lesson): a leaf UI must never gate the chain. Move
all Headlamp resources into a new non-gating Flux Kustomization
clusters/base/headlamp.yaml (dependsOn platform-config, wait:false, SOPS
decryption patched per-env in clusters/dev) — mirroring oauth2-proxy exactly.
The new Kustomization owns the whole platform/headlamp component; its HTTPRoute
moves from platform-config into platform/headlamp/kustomization.yaml. Removed
headlamp from platform/kustomization.yaml and its httproute from
platform-config/kustomization.yaml. Now a crashlooping Headlamp can never stall
platform/platform-config/apps.

Part B (make it start): pin the chart to 0.41.0 (`>=0.41.0 <0.42.0`). The
backend added `f.Int("session-ttl", …)` in v0.41.0 and chart 0.41.0 pins that
same image (appVersion == chart version), so chart+image realign; every arg the
chart renders for our values is supported. OIDC config + per-user RBAC
(clusterRoleBinding.create:false) unchanged.

Gate: `make -C ci gate` PASS.

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

clusters/dev/kustomization.yaml

@@ -77,6 +77,20 @@ patches:
           provider: sops
           secretRef:
             name: sops-age
+  # headlamp moved to its own post-platform Flux Kustomization (clusters/base/headlamp.yaml).
+  # It carries headlamp-oidc-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: headlamp
+        namespace: flux-system
+      spec:
+        decryption:
+          provider: sops
+          secretRef:
+            name: sops-age
   - patch: |-
       apiVersion: source.toolkit.fluxcd.io/v1
       kind: GitRepository