fix part 3

backend/app/database/models/comment.py

@@ -1,9 +1,8 @@
 """Comment model for board annotations."""
 
 import uuid
-from datetime import datetime, timezone
 
-from sqlalchemy import Boolean, Column, DateTime, ForeignKey, String, Text
+from sqlalchemy import Boolean, Column, DateTime, ForeignKey, String, Text, func
 from sqlalchemy.dialects.postgresql import JSONB, UUID
 from sqlalchemy.orm import relationship
 
@@ -21,7 +20,7 @@ class Comment(Base):
     author_name = Column(String(100), nullable=False)
     content = Column(Text, nullable=False)
     position = Column(JSONB, nullable=True)  # Optional canvas position reference
-    created_at = Column(DateTime, nullable=False, default=lambda: datetime.now(timezone.utc))
+    created_at = Column(DateTime, nullable=False, server_default=func.now())
     is_deleted = Column(Boolean, nullable=False, default=False)
 
     # Relationships