Delete Deal

Delete a deal.

THE TOMBSTONE RULE (#16242): deal.deleted has to be self-contained, because by the time a consumer reads it the row is gone and deal_id resolves to nothing. The payload therefore comes from RETURNING on the DELETE itself — one round trip, and no window in which a concurrent write could change what a preceding SELECT had read. background_tasks is injected (not a body field, so the OpenAPI schema is unchanged) for the same reason as on PATCH.