Function delete_relationships_by_relation 

pub fn delete_relationships_by_relation(
    conn: &Connection,
    namespace: &str,
    relation: &str,
) -> Result<(usize, Vec<i64>), AppError>

Deletes all relationships matching a relation type within a namespace.

Operates in chunks of 1000 to avoid holding long write locks and blocking WAL readers. After deletion, recalculates degree for every affected entity.

Returns (count_deleted, affected_entity_ids).

Errors

Returns Err(AppError::Database) on any rusqlite failure.