Migrations that lock the table

Split it: add the column nullable, backfill in batches, add a validated check constraint, then set NOT NULL. Each step takes a brief lock instead of one long one, and the backfill can be paused if replication lag grows.

For monitoring, read reltuples from pg_class, which the planner keeps roughly current. It is off by a few percent between vacuums, which is fine for a dashboard and wrong for invoicing.