mailu: the queue is root's and traversable, which is postfix's own convention

The declared 0700 was applied at take and broke mail quietly: postfix's
master runs as root but pickup and smtpd drop to uid postfix, and a
spool root they cannot traverse is a maildrop they cannot scan and a
rewrite socket they cannot open — auth succeeded and MAIL FROM hung.
0755 root is exactly what postfix's own set-permissions makes of
/var/spool/postfix. Fixed live by chmod first; declared here so the
next push stops undoing it.

modules/mailu/module.json

@@ -211,7 +211,7 @@
       "id": "data-mailqueue",
       "type": "directory",
       "path": "/services/mailu/data/mailqueue",
-      "mode": "0700"
+      "mode": "0755"
     },
     {
       "id": "data-filter",