fix(apiserver): ArkConfig is unserved in postgres mode, so cluster-wide defaults silently don't apply

[pontino]

Problem

In postgres mode, ArkConfig does not exist as an API resource, and the admission path that would consume it passes a nil lookup. Every cluster-wide default routed through ArkConfig silently does nothing in postgres mode — no error, no warning, just etcd/postgres behavioral divergence.

• ark/internal/apiserver/server.go:94-95 puts ArkConfig/ArkConfigList in the Scheme, but V1Alpha1Resources (ark/internal/apiserver/resources.go:21) omits them → no REST endpoint. Postgres mode skips CRDs entirely, so the resource is uncreatable, and ark-api's arkconfig.py endpoints fail underneath.
• ark/internal/apiserver/admission.go:31,47,61 all call validation.ApplyDefaults(ctx, obj, nil), so DefaultQuery never injects spec.ttl from ArkConfig.spec.queryTTL.
• The gap is mechanical: WebhookLookup implements ArkConfigLookup.GetArkConfig; StorageLookup (postgres mode) does not — hence the nil.

Why this matters more after #3307

#3307 makes ArkConfig the injection mechanism for the cluster default Memory, through the same DefaultQuery/ArkConfigLookup path. Once it merges, postgres mode won't just miss queryTTL — the whole default-memory feature will silently not exist there. That settles the old "wire it vs delete it" question from the #2627 review thread: wire it.

Proposed scope

1. Serve ArkConfig from the aggregated apiserver: add it to V1Alpha1Resources. It would be the first cluster-scoped kind there (ResourceDef has no scope field; the registry assumes namespaced), so this includes a ClusterScoped flag threaded through GenericStorage/discovery. I'll spike this piece first and report back if it's bigger than it looks.
2. Implement StorageLookup.GetArkConfig via Backend.Get(ctx, "ArkConfig", "", "default"), honoring the existing contract (any error ⇒ fall back to hardcoded defaults, never block admission).
3. Replace the three nils in admission.go with the lookup — after which defaulting behaves identically in both modes, including feat(controller): add ArkConfig.defaultMemory and inject spec.memory at admission #3307's defaultMemory once merged.
4. Separate commit: delete the dead storage.postgresql.* block from ark/dist/chart/values.yaml — consumed by zero templates (only .Values.storage.backend is read; the real knobs live on chart-apiserver). Documented as dead in the feat(apiserver): support Postgres TLS verification and custom CA bundle #2627 review; this closes that loop.

I'll pick this up. cc @Nab-0 @skazinka @CDimonaco