Function supersede 

pub async fn supersede(
    pool: &PgPool,
    actor: &Actor,
    item_id: Uuid,
    successor_number: &str,
    note: &str,
) -> AppResult<()>

Declares item_id superseded by successor_number (replacing an earlier successor).

Invariants

• Permission: both items visible to the actor (“not found” otherwise); CAD editor and member of the group of item_id (403). An item cannot supersede itself (400). A group with obsolete_requires_change refuses the direct supersession (409): it goes through a change order.
• Locks: the table item_supersessions (SHARE ROW EXCLUSIVE), so two concurrent supersessions cannot close a circle unseen; a circle is a 409.
• Audit: item.supersede.
• Notifications: the watchers of item_id (also when the group does not require a change order); people who may not see the successor get a text without its number. Webhooks: none.
• Data: an earlier successor of the item is replaced.
• Atomic: one transaction.