refactor: use playlist item instead of entry

src/aura_engine/core/channels.py

src/aura_engine/engine.py

src/aura_engine/events.py

src/aura_engine/plugins/clock.py

src/aura_engine/resources.py

src/aura_engine/scheduling/domain.py

src/aura_engine/scheduling/scheduler.py

src/aura_engine/scheduling/utils.py