feat(grafana): provision disk alerts to ntfy

Provision Grafana Unified Alerting as code: ntfy webhook contact point
(?template=grafana), notification policy tree, and cumulative disk tiers
for HDD temperature (48-60C, <20C) plus ZFS/filesystem capacity. Lower the
smartctl exporter interval to 30m for alert responsiveness.

roles/grafana/files/alerting-policies.yaml

@@ -0,0 +1,16 @@
+apiVersion: 1
+
+# Managed by Ansible (roles/grafana). Notification policy tree.
+# NOTE: provisioning replaces the entire tree; UI edits are overwritten on deploy.
+policies:
+  - orgId: 1
+    receiver: ntfy-sys-alerts
+    group_by:
+      - alertname
+      - instance
+      - device
+      - pool
+      - mountpoint
+    group_wait: 30s
+    group_interval: 5m
+    repeat_interval: 1h