Database migration checklist

Fifteen checks around a schema or data migration, ordered by when the damage becomes expensive to undo. The first group runs before anything is applied, because a migration you cannot reverse and a backup you have never restored are the same risk wearing different names. The second group runs on the copy, where a wrong answer costs nothing. The third group runs after the migration lands, and it exists because the failures that follow a migration are quiet ones: a constraint that was never enforced, an index the planner stopped using, a timestamp column that changed meaning. Every check on this list was written against SQLite on one machine, and each page names the client it ran through, because two clients reading the same file disagree more often than the documentation suggests.