feat: deal status history and default created_at in db

models/mixins.py

@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone
 
 from sqlalchemy import DateTime
 from sqlalchemy.orm import Mapped, mapped_column
@@ -17,5 +17,7 @@ class SoftDeleteMixin:
 
 class CreatedAtMixin:
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), nullable=False
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        nullable=False,
     )