Fix celery-beat crash: write schedule to /tmp tmpfs

The read_only filesystem prevents celery-beat from writing its
schedule file to /app. Redirect to /tmp which is a writable tmpfs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -155,7 +155,7 @@ services:
       - /tmp
     networks:
       - rfiles-internal
-    command: celery -A config beat --loglevel=info
+    command: celery -A config beat --loglevel=info --schedule=/tmp/celerybeat-schedule
 
 volumes:
   rfiles_postgres_data: