Loosen up backup rules

The git.sr.ht backups tend to take a pretty long time these days and we
get some false positives on this.

Might tune this figure back down a bit if/when we switch to bupstash.

backup_rules.yml

@@ -3,12 +3,12 @@ groups:
 - name: backup
   rules:
   - alert: Backup age
-    expr: time() - last_backup > 60 * 60 * 72
+    expr: time() - last_backup > 60 * 60 * 168
     for: 2m
     labels:
       severity: important
     annotations:
-      summary: "{{ $labels.instance }} last backup age is >48 hours"
+      summary: "{{ $labels.instance }} last backup age exceeds 1 week"
   - alert: ZFS snapshot age
     expr: time() - zfs_last_snapshot > 60 * 60 * 24
     for: 2m