Replace hardcoded number by a config value

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

@@ -17,6 +17,13 @@ docspell.server {
     port = 7880
   }
 
+  # This is a hard limit to restrict the size of a batch that is
+  # returned when searching for items. The user can set this limit
+  # within the client config, but it is restricted by the server to
+  # the number defined here. An admin might choose a lower number
+  # depending on the available resources.
+  max-item-page-size = 500
+
   # Authentication.
   auth {