Give management-platform a scoped ServiceAccount for k8s backup/restore

The pod running app.py had no kubectl binary and only a read-only SA
(management-platform-viewer-sa) — backup-k8s-apps.sh/restore-k8s-apps.sh
could never actually run from inside it. Adds:

- management-platform-backup-role (ClusterRole, bound via RoleBinding only
  in n8n/odoo/mautic/nextcloud/erpnext — not cluster-wide): get/list/watch
  on pods/deployments/configmaps/ingresses/PVCs, pods/exec create, pods
  create+delete (needed for restore's populate-before-scale-up loader pod),
  secrets get/list/update/patch, deployments/scale update/patch only (no
  write on full Deployment/Service/ConfigMap/Ingress specs).
- management-platform-sa, replacing management-platform-viewer-sa as the
  pod's identity — inherits the old viewer-role's read-only bindings too
  (retargeted in management-platform-viewer-rbac.yaml) so the existing
  cluster-view page keeps working under one SA.
- k3s binary hostPath-mounted read-only into the pod (same pattern as the
  Jenkins agent's docker-cli container), so `k3s kubectl` is available
  where no separate kubectl binary exists.

Both scripts updated to fall back to k3s kubectl when no kubectl is on
PATH, and to use /proc/meminfo instead of `free` for the resource-safety
check (not present in the pod's minimal image). Also fixes a real issue
found while live-testing this from inside the pod: the pod's pre-existing
/root hostPath mount also exposes the host's own admin ~/.kube/config,
which k3s kubectl was silently preferring over the scoped SA token —
forcing --kubeconfig=/dev/null in both scripts closes that.

Verified end-to-end from inside the actual management-platform pod:
manifests/secret/DB-dump/PVC-data backup for n8n succeeds under the new
SA's scoped permissions, and a kube-system access attempt is correctly
rejected (Forbidden) once the kubeconfig leak is closed.

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

k8s/management-platform-deployment.yaml

@@ -22,7 +22,7 @@ spec:
         app.kubernetes.io/managed-by: navitrends-pfe-migration
         owner: ameni-boukattaya
     spec:
-      serviceAccountName: management-platform-viewer-sa
+      serviceAccountName: management-platform-sa
       containers:
       - name: management-platform
         image: management-platform:latest
@@ -47,6 +47,9 @@ spec:
         - mountPath: /app/config.py
           name: config-py
           readOnly: true
+        - mountPath: /usr/local/bin/k3s
+          name: k3s-bin
+          readOnly: true
       volumes:
       - name: docker-sock
         hostPath:
@@ -68,3 +71,7 @@ spec:
         hostPath:
           path: /root/management-platform/config.py
           type: File
+      - name: k3s-bin
+        hostPath:
+          path: /usr/local/bin/k3s
+          type: File