Function create_or_fetch_relationship 

pub fn create_or_fetch_relationship(
    conn: &Connection,
    namespace: &str,
    source_id: i64,
    target_id: i64,
    relation: &str,
    weight: f64,
    description: Option<&str>,
) -> Result<(i64, bool), AppError>

Creates a relation if it does not exist (returns action=“created”) or returns the existing relation (action=“already_exists”) with updated weight.