add created_at field to state model

services/backend/apps/competition/models.py

@@ -1,3 +1,5 @@
+from datetime import datetime
+
 from django.db import models
 
 from apps.core.models import BaseModel
@@ -49,3 +51,4 @@ class State(BaseModel):
     user = models.ForeignKey(User, on_delete=models.CASCADE)
     competition = models.ForeignKey(Competition, on_delete=models.CASCADE)
     state = models.CharField(choices=StateChoices.choices, max_length=11)
+    changed_at = models.DateTimeField(default=datetime.now)