v-m/alerts: Add Paperless-ngx email task alert

This alert should fire if the background task to fetch e-mail and import
them into Paperless-ngx has not run for a while.

victoria-metrics/alerts.yml

@@ -183,3 +183,17 @@ groups:
         Failing Celery tasks may indicate a problem with the Paperless-ngx
         deployment and can result in data loss.  Check the Paperless-ngx logs
         for details about the task failures.
+  - alert: Paperless email task not running
+    expr: >-
+      absent(
+        flower_events_total{
+          type="task-started",
+          task="paperless_mail.tasks.process_mail_accounts"
+        }
+      )
+    annotations:
+      summary: Paperless task to process mail accounts has not run recently
+      description: >-
+        Paperless-ngx uses a scheduled Celery task to periodically poll email
+        mailboxes for new messages.  If this task does not start, new email
+        messages will not be downloaded and imported into the document library.