fix: trigger legacy tasks manager every 5m (#2987)

### Description

This ensures that when the LibreTime interface is not used for a long
time, we still have the task manager run essential tasks for the
schedule.

Related to #2670

worker/install/systemd/libretime-worker.service

@@ -25,6 +25,7 @@ WorkingDirectory=@@WORKING_DIR@@/worker
 ExecStart=/usr/bin/sh -c '@@VENV_DIR@@/bin/celery worker \
     --app=libretime_worker.tasks:worker \
     --config=libretime_worker.config \
+    --beat \
     --time-limit=1800 \
     --concurrency=1 \
     --loglevel=INFO \