feat(worker): load config using shared helpers

BREAKING CHANGE: The worker `RMQ_CONFIG_FILE` environement variable has
been renamed to `LIBRETIME_CONFIG_FILEPATH`. In addition the systemd
working directory for the worker has changed from `/srv/airtime` to
`/var/lib/libretime/worker`.

worker/libretime_worker/config.py

@@ -0,0 +1,32 @@
+from os import getenv
+
+from kombu import Exchange, Queue
+from libretime_shared.config import BaseConfig, RabbitMQConfig
+
+
+class Config(BaseConfig):
+    rabbitmq: RabbitMQConfig = RabbitMQConfig()
+
+
+LIBRETIME_CONFIG_FILEPATH = getenv("LIBRETIME_CONFIG_FILEPATH")
+
+config = Config(filepath=LIBRETIME_CONFIG_FILEPATH)
+
+# Celery amqp settings
+BROKER_URL = config.rabbitmq.url
+CELERY_RESULT_BACKEND = "amqp"  # Use RabbitMQ as the celery backend
+CELERY_RESULT_PERSISTENT = True  # Persist through a broker restart
+CELERY_TASK_RESULT_EXPIRES = 900  # Expire task results after 15 minutes
+CELERY_RESULT_EXCHANGE = "celeryresults"  # Default exchange - needed due to php-celery
+CELERY_QUEUES = (
+    Queue("podcast", exchange=Exchange("podcast"), routing_key="podcast"),
+    Queue(exchange=Exchange("celeryresults"), auto_delete=True),
+)
+CELERY_EVENT_QUEUE_EXPIRES = 900  # RabbitMQ x-expire after 15 minutes
+
+# Celery task settings
+CELERY_TASK_SERIALIZER = "json"
+CELERY_RESULT_SERIALIZER = "json"
+CELERY_ACCEPT_CONTENT = ["json"]
+CELERY_TIMEZONE = "Europe/Berlin"
+CELERY_ENABLE_UTC = True