feat(zfs): add capacity metrics and Grafana dashboard

Deploy an hourly systemd timer that writes pool and dataset capacity to
node_exporter's textfile collector, since --collector.zfs exposes only
ARC/I-O/state. Add a provisioned 'ZFS - Pools & Datasets' dashboard with
per-pool totals, per-dataset used/quota and daily growth panels.

roles/grafana/tasks/main.yml

@@ -260,6 +260,15 @@
     - systemd-dashboard.json
   notify: Restart grafana
 
+- name: Deploy ZFS dashboard
+  ansible.builtin.copy:
+    src: zfs-dashboard.json
+    dest: "{{ grafana_provisioning_path }}/dashboards/zfs.json"
+    owner: "{{ grafana_user }}"
+    group: "{{ grafana_group }}"
+    mode: "0644"
+  notify: Restart grafana
+
 # Service management
 - name: Enable and start Grafana service
   ansible.builtin.systemd: