fix(celery): use separate beat for single process

Mixing these two together might cause issues observed in #3738.

Author: nijel

etc/supervisor/conf.d/celery-single.conf

@@ -1,6 +1,6 @@
 [program:celery]
 environment = CELERY_WORKER_RUNNING=1,CELERY_APP=weblate.utils
-command = celery --app=weblate.utils worker --beat --queues=celery,notify,memory,translate,backup --pool=solo %(ENV_CELERY_SINGLE_OPTIONS)s
+command = celery --app=weblate.utils worker --queues=celery,notify,memory,translate,backup --pool=solo %(ENV_CELERY_SINGLE_OPTIONS)s
 autorestart = true
 stdout_logfile=/dev/fd/1
 stdout_logfile_maxbytes=0

start

@@ -217,6 +217,7 @@ if [ "$1" = "runserver" ]; then
         if [ "${CELERY_SINGLE_PROCESS:-0}" -eq 1 ]; then
             # Symlink single process service only
             ln -s /etc/supervisor/conf.d/celery-single.conf "$SUPERVISOR_CONF"
+            ln -s /etc/supervisor/conf.d/celery-beat.conf "$SUPERVISOR_CONF"
         else
             # Symlink all celery services but the single process
             find /etc/supervisor/conf.d -type f -name 'celery-*.conf' ! -name 'celery-single.conf' -print0 | xargs -0 -I '{}' ln -s '{}' "$SUPERVISOR_CONF"