Delete Campaign

Delete a campaign and its placements.

Attached videos survive. Tracking a video and having paid for it are different facts, and losing the tracking because the bookkeeping was tidied would be genuine data loss.

The detach is done here rather than left to the column's ON DELETE SET NULL. That clause describes the model, but placement_id reaches the deployed table through ALTER TABLE ... ADD COLUMN in run_alter_migrations, which adds no constraint — so in production there is no foreign key to act on and the videos would be left pointing at placements that no longer exist. Doing it explicitly is the only version that is true on both the model and the live database.