Add read-only Kubernetes cluster view alongside Docker container view

New modules/kubernetes.py lists pods and deployments across the 8 app
namespaces via the in-cluster management-platform-viewer-sa (read-only,
no secrets/exec/log), with metrics-server usage as a best-effort extra
that degrades silently when unreachable. New /cluster page and
/api/cluster endpoint, nav entry, and templates render pods/deployments
grouped by namespace using the existing card/badge design system.
Additive only — no existing Docker routes or views touched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

platform/requirements.txt

@@ -1,3 +1,4 @@
 boto3==1.43.5
 flask
-psutil
+psutil
+kubernetes