Function enqueue_mutation 

pub(crate) async fn enqueue_mutation<S: DatabaseService>(
    db_service: &S,
    user_id: String,
    operation: String,
    item_id: Option<String>,
    payload: Option<String>,
) -> Result<i64, String>

Insert one pending mutation and return the id of that row.

TRACES: UR-002, UR-017 | DR-014