Integrate periodic tasks

The first use case for periodic task is the cleanup of expired
invitation keys. This is part of a house-keeping periodic task.

modules/joex/src/main/resources/reference.conf

@@ -77,6 +77,21 @@ docspell.joex {
     wakeup-period = "10 minutes"
   }
 
+  # Docspell uses periodic house keeping tasks, like cleaning expired
+  # invites, that can be configured here.
+  house-keeping {
+
+    # When the house keeping task executes. Default is every 4 hours.
+    schedule = "*-*-* 0/4:00:00"
+
+    # This task removes invitation keys that have been created but not
+    # used. The timespan here must be greater than the `invite-time'
+    # setting in the rest server config file.
+    cleanup-invites = {
+      older-than = "30 days"
+    }
+  }
+
   # Configuration of text extraction
   extraction {
     # For PDF files it is first tried to read the text parts of the