ALTER TABLE accounts ADD COLUMN folder_mapping   JSONB NOT NULL DEFAULT '{}';
ALTER TABLE accounts ADD COLUMN excluded_folders JSONB NOT NULL DEFAULT '[]';

-- Carry the existing task-wide mapping onto its accounts so behavior is preserved.
UPDATE accounts a SET folder_mapping = t.folder_mapping
  FROM tasks t WHERE a.task_id = t.id AND t.folder_mapping <> '{}'::jsonb;
