Become a nox mesh module: smtp granted, the shared channel accepted

The mailu smtp grant replaces the hand-carried mailbox credential — the
module contributes account amqp-forwarder and composes its login from
the binding, the de-spiegel pattern. The AMQP side stays the operator's
on purpose: EMAILDELIVERY_T is a vhost external publishers share, which
the consumer-owned-vhost provision deliberately cannot express, so the
URL rides as an accepted secret. Built from source; the old image's
registry no longer exists.

Dockerfile

@@ -1,4 +1,6 @@
-FROM node:22-alpine
+# The base arrives pinned from module.json's build.on.
+ARG NODE_BASE
+FROM ${NODE_BASE}
 WORKDIR /app
 COPY package.json ./
 RUN npm install --production