fix: Add author field to ContentSerializer for Forums V2 API compatibility (#245)

The Forums V2 API responses were missing the 'author' attribute that is expected by
the frontend, causing issues when displaying comment/thread author information.

Analysis showed that:
- The ContentSerializer already has 'username' field mapped to 'author_username'
- Frontend expects an 'author' field in addition to 'username'
- Both should reference the same source data (author_username)

This change adds the 'author' field to the DRF serializer, mapped to the same
'author_username' source as 'username', maintaining backward compatibility while
fixing the missing field issue.

Fixes compatibility issues with Forums V2 API responses.

Author: blarghmatey

forum/serializers/contents.py

@@ -67,6 +67,9 @@ class ContentSerializer(serializers.Serializer[dict[str, Any]]):
     at_position_list = serializers.ListField(default=[])
     user_id = serializers.CharField(source="author_id")
     username = serializers.CharField(source="author_username", allow_null=True)
+    author = serializers.CharField(
+        source="author_username", allow_null=True, required=False
+    )
     commentable_id = serializers.CharField(default="course")
     votes = VoteSummarySerializer()
     abuse_flaggers = serializers.ListField(child=serializers.CharField(), default=[])