monitoring(pve-exporter): mount emptyDir at /tmp for Python tempfile

readOnlyRootFilesystem=true leaves no writable directory under /tmp,
and pve_exporter's Python startup fails with FileNotFoundError from
tempfile._get_default_tempdir. Mount a small emptyDir at /tmp.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

kubernetes/apps/monitoring/pve-exporter-deployment.yaml

@@ -52,10 +52,15 @@ spec:
             - name: config
               mountPath: /etc/pve-exporter
               readOnly: true
+            # Python needs a writable temp dir; the rootfs is read-only.
+            - name: tmp
+              mountPath: /tmp
       volumes:
         - name: config
           secret:
             secretName: pve-exporter-config
+        - name: tmp
+          emptyDir: {}
 ---
 apiVersion: v1
 kind: Service