Add Garage S3-compatible object store to platform

Cluster-local object store for build artifacts (CLI binaries etc.)
so Docker builds don't depend on flaky external downloads.

- Single-node Garage v1.0.1 StatefulSet (LMDB, replication=1)
- Metadata on longhorn-nvme (1Gi), data on longhorn HDD (20Gi)
- S3 API at garage.platform.svc:3900
- External ingress at s3.coreworlds.io (internal-only)
- SealedSecret for admin token and RPC secret

infra/kubernetes/platform/traefik/kustomization.yaml

@@ -9,5 +9,6 @@ resources:
   - ingressroute-longhorn.yaml
   - ingressroute-harness.yaml
   - ingressroute-gitea.yaml
+  - ingressroute-garage.yaml
   - certificate-internal.yaml
   - servicemonitor.yaml