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>

backup/backup-k8s-apps.sh

@@ -26,6 +26,26 @@
 
 set -uo pipefail
 
+# When run inside the management-platform pod, only the static k3s binary is
+# hostPath-mounted (no separate kubectl binary) — fall back to `k3s kubectl`.
+# IMPORTANT: this pod also has a pre-existing rw hostPath mount of /root (for
+# unrelated file-browsing features), which means the host's own root-owned
+# ~/.kube/config (full cluster-admin) is silently visible inside the pod too.
+# `--kubeconfig=/dev/null` is REQUIRED here to stop k3s kubectl from picking
+# that up as a base config — without it, every command below silently runs
+# as cluster-admin instead of the scoped management-platform-sa token,
+# defeating the RBAC in management-platform-backup-rbac.yaml entirely.
+if ! command -v kubectl &>/dev/null && command -v k3s &>/dev/null; then
+    kubectl() {
+        k3s kubectl \
+            --kubeconfig=/dev/null \
+            --server=https://kubernetes.default.svc \
+            --certificate-authority=/var/run/secrets/kubernetes.io/serviceaccount/ca.crt \
+            --token="$(cat /var/run/secrets/kubernetes.io/serviceaccount/token)" \
+            "$@"
+    }
+fi
+
 # --------------------------------------------------
 # Parse --apps
 # --------------------------------------------------
@@ -132,7 +152,9 @@ log_status() {
 # --------------------------------------------------
 check_resources() {
     local avail_mb avail_gb
-    avail_mb=$(free -m | awk '/^Mem:/{print $7}')
+    # /proc/meminfo instead of `free` — `free` isn't present in the
+    # management-platform pod's minimal image, /proc/meminfo always is.
+    avail_mb=$(awk '/^MemAvailable:/{printf "%d", $2/1024}' /proc/meminfo)
     avail_gb=$(df -BG / | awk 'NR==2{gsub("G","",$4); print $4}')
     if [ "$avail_mb" -lt 1536 ]; then
         echo "   ❌ RAM available ${avail_mb}MB < 1536MB threshold — aborting."