Direct database deletes should still cascade to avoid orphaned records.
The migration now only adds the three deleted_at columns without touching
the existing FK constraints.
Replace hard deletes with deletedAt timestamp flags on all three entities.
Deleting a client or project only sets its own deletedAt; child records are
excluded implicitly by filtering on parent deletedAt in every read query.
Raw SQL statistics queries also filter out soft-deleted parents.
FK ON DELETE CASCADE removed from Project→Client and TimeEntry→Project.