6869: Clarify local-only dev credentials in docker-compose.override

Replace opaque RabbitMQ Erlang cookie with self-documenting value
and add comment indicating these are local dev credentials.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: turegjorup

docker-compose.override.yml

@@ -9,7 +9,7 @@ services:
     environment:
       - RABBITMQ_DEFAULT_USER=user
       - RABBITMQ_DEFAULT_PASS=password
-      - RABBITMQ_ERLANG_COOKIE='d53f219cd9376f8f440aaf9889f315ab'
+      - RABBITMQ_ERLANG_COOKIE='local-dev-cookie-not-a-secret'
     healthcheck:
       test: rabbitmq-diagnostics check_port_connectivity
       interval: 1s