fix(api): move gunicorn worker config to python file (#2854)

### Description

Passing the --lifespan flag to gunicorn does not forward the option to
uvicorn.

api/install/systemd/libretime-api.service

@@ -25,8 +25,7 @@ Type=notify
 KillMode=mixed
 ExecStart=@@VENV_DIR@@/bin/gunicorn \
         --workers 4 \
-        --worker-class uvicorn.workers.UvicornWorker \
-        --lifespan off \
+        --worker-class libretime_api.gunicorn.Worker \
         --log-file - \
         --bind unix:/run/libretime-api.sock \
         libretime_api.asgi