novox conversions: short slugs for the login-length cap + real roundcube digest

The whole-mesh-novox lab validation found three modules unresolvable: a
provision-consuming module's minted login is mesh_<node>_<module>, and on novox
`mesh_novox_only_office`(22)/`_de_spiegel`(21)/`_amqp_email_forwarder`(31)
overflow the 20-char S3 access-key cap (ADR 0049). Added a short `slug` each
(office/spiegel/emailfwd → 17/18/19 chars). Also fixed mailu's roundcube image:
the pinned digest returns "manifest unknown" from ghcr; corrected to the real
:1.9 digest.

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

modules/amqp-email-forwarder/module.json

@@ -1,6 +1,7 @@
 {
   "module": "amqp-email-forwarder",
   "version": "1",
+  "slug": "emailfwd",
   "capabilities": [
     "container-runtime"
   ],

modules/de-spiegel/module.json

@@ -1,6 +1,7 @@
 {
   "module": "de-spiegel",
   "version": "1",
+  "slug": "spiegel",
   "capabilities": [
     "container-runtime"
   ],

modules/mailu/module.json

@@ -303,7 +303,7 @@
       "id": "webmail",
       "type": "container",
       "name": "mailu-webmail",
-      "image": "ghcr.io/mailu/roundcube@sha256:076b720fc766e58a97321cdb700e887c2008d6d323685fe59f323088333059dc",
+      "image": "ghcr.io/mailu/roundcube@sha256:19ccc9c21b2420dabb893ffa707ef90785c785e53dcb6bb9f98da01598412c43",
       "network": "mailu",
       "env-file": [
         "/var/lib/mailu/mailu.env",

modules/only-office/module.json

@@ -1,6 +1,7 @@
 {
   "module": "only-office",
   "version": "1",
+  "slug": "office",
   "capabilities": [
     "container-runtime"
   ],