Function execute_loopback 

pub async fn execute_loopback(
    pool: &DbPool,
    node_id: i64,
    tweet_id: &str,
    url: &str,
    content_type: &str,
) -> LoopBackResult

Execute provenance-driven loop-back: look up the source note for a content node and write publishing metadata into its YAML front-matter.

Returns LoopBackResult indicating the outcome. DB lookup failures are logged and mapped to result variants rather than propagated as errors.