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/templates/base.html

@@ -35,6 +35,9 @@
                     <i class="fas fa-cubes"></i><span>All Containers</span>
                     <span class="nav-badge" id="nav-badge-containers">—</span>
                 </a>
+                <a class="nav-item {% if active_page == 'cluster' %}active{% endif %}" href="{{ url_for('cluster_page') }}">
+                    <i class="fas fa-diagram-project"></i><span>Cluster</span>
+                </a>
 
                 <div class="nav-section-label" style="margin-top:20px;">OPERATIONS</div>
                 <a class="nav-item {% if active_page == 'restore' %}active{% endif %}" href="{{ url_for('restore_page') }}">