ALTER TABLE public.ak_assignments
  ADD COLUMN project_name VARCHAR(128), -- 作业所属项目/主题
  ADD COLUMN status VARCHAR(32) DEFAULT 'active', -- 作业状态
  ADD COLUMN start_time TIMESTAMP WITH TIME ZONE, -- 开始时间
  ADD COLUMN end_time TIMESTAMP WITH TIME ZONE; -- 结束时间

COMMENT ON COLUMN public.ak_assignments.project_name IS '作业所属项目/主题';
COMMENT ON COLUMN public.ak_assignments.status IS '作业状态';
COMMENT ON COLUMN public.ak_assignments.start_time IS '开始时间';
COMMENT ON COLUMN public.ak_assignments.end_time IS '结束时间';