Function extend_corpus 

pub fn extend_corpus(
    name: &str,
    actor: Actor,
    pool: &State<DbPool>,
    database_url: &State<DatabaseUrl>,
) -> Result<(Status, Json<JobDto>), Status>

Extends an existing corpus with newly-arrived entries; starts an in-process job and returns 202 Accepted + the job handle. Token-gated via the Actor guard; 401 without a valid token, 404 if the corpus is unknown.