fix: register problem tasks with celery

Author: magnified103

judge/tasks/__init__.py

@@ -1,6 +1,7 @@
 from judge.tasks.contest import *
 from judge.tasks.demo import *
 from judge.tasks.organization import *
+from judge.tasks.problem import *
 from judge.tasks.submission import *
 from judge.tasks.user import *
 from judge.tasks.webhook import *