-- 0019 的回滚：先删索引（否则 SQLite 拒绝 DROP COLUMN），再删两张新表，
-- 最后按逆序删掉 memory_items 的整理列。
DROP INDEX IF EXISTS idx_memory_tidy_runs_started_at;
DROP TABLE IF EXISTS memory_tidy_runs;
DROP INDEX IF EXISTS idx_memory_tidy_candidates_status;
DROP TABLE IF EXISTS memory_tidy_candidates;
DROP INDEX IF EXISTS idx_memory_items_status;
ALTER TABLE memory_items DROP COLUMN expires_at;
ALTER TABLE memory_items DROP COLUMN valid_to;
ALTER TABLE memory_items DROP COLUMN occurred_at;
ALTER TABLE memory_items DROP COLUMN merge_count;
ALTER TABLE memory_items DROP COLUMN merged_from;
ALTER TABLE memory_items DROP COLUMN merged_into;
ALTER TABLE memory_items DROP COLUMN status;