Keep RabbitMQ data on recreate and proxy to unique container names

RabbitMQ stores its data under rabbit@<hostname>, and the hostname
changed with every recreated container, so an image update or an .env
change came up with an empty broker. Pin it to `rabbitmq` and document
how to carry the definitions over on an existing deployment.

Every project shares infra-net, where a generic service name like `api`
resolves to every container that has it, round-robin, across projects.
The edge now proxies to container names (musify-api, infra-zitadel, ...).

compose.yml

@@ -192,6 +192,10 @@ services:
   rabbitmq:
     image: rabbitmq:4-management-alpine
     container_name: infra-rabbitmq
+    # The node is named rabbit@<hostname> and its data lives under that name.
+    # Without a fixed hostname every recreated container gets a new one and
+    # starts on an empty data directory (users, vhosts, queues all gone).
+    hostname: rabbitmq
     environment:
       RABBITMQ_DEFAULT_USER: ${RABBITMQ_DEFAULT_USER}
       RABBITMQ_DEFAULT_PASS: ${RABBITMQ_DEFAULT_PASS}