v-m/alerts: Improve Paperless-ngx Celery task alert

The `flower_events_total` metric is a counter, so its value only ever
increases (discounting restarts of the server process).  As such,
nonzero values do not necessarily indicate a _current_ problem, but
rather that there was one at some point in the past.  To identify
current issues, we need to use the `increase` function, and then apply
the `max_over_time` function so that the alert doesn't immediately reset
itself.

victoria-metrics/alerts.yml

@@ -172,9 +172,13 @@ groups:
   rules:
   - alert: Celery tasks failed
     expr: >-
-      flower_events_total{job="paperless-ngx", type="task-failed"} > 0
+      max_over_time(
+        increase(
+          flower_events_total{job="paperless-ngx", type="task-failed"}
+        )[24h]
+      ) > 0
     annotations:
-      summary: One or more Celery tasks have failed
+      summary: Paperless-ngx Celery task failed
       description: >-
         Failing Celery tasks may indicate a problem with the Paperless-ngx
         deployment and can result in data loss.  Check the Paperless-ngx logs