fix: handle None author_id in step sync to avoid invalid UUID error

When a system/default tree has no author (author_id is None),
str(None) produces the literal string 'None' which asyncpg
rejects as an invalid UUID for the created_by column.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/core/step_sync.py

@@ -157,7 +157,7 @@ async def sync_steps_from_tree(
                 "title": step_data["title"],
                 "step_type": step_data["step_type"],
                 "content": json.dumps(step_data["content"]),
-                "created_by": str(author_id),
+                "created_by": str(author_id) if author_id else None,
                 "account_id": str(account_id) if account_id else None,
                 "visibility": visibility,
                 "source_tree_id": str(tree_id),