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/base/kustomization.yaml

@@ -21,4 +21,9 @@ resources:
   # 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)
+  # headlamp dashboard — POST-platform tier (dependsOn platform-config, wait:false). A leaf UI whose
+  # HelmRelease can crashloop (upstream headlamp-0.40.1 `-session-ttl` chart/image desync) must NOT
+  # sit in the gating `platform` tier, where its unhealth stalled platform-config + oauth2-proxy +
+  # every apps wave/vm on "dependency not ready" (DECISIONS-LOG: "Headlamp gating stall").
+  - headlamp.yaml        # Flux Kustomization -> ./platform/headlamp (dependsOn platform-config)
   - cluster-vars.yaml    # overlay knob surface (base defaults)