Instances that accumulated project managers, favorites, board memberships
or board subscriptions pointing at a project or board that no longer exists
had no way to find or clear them.
The script reports by default and only deletes with `--apply`, which first
asks whether a backup exists and refuses outright without a terminal to ask
on. It removes nothing but reference rows: structural orphans such as a
board without a project carry user content, and a membership whose
denormalized project is stale still grants real access to an existing
board, so both are reported and left alone.
