Function activate_on_corpus_page 

pub fn activate_on_corpus_page(
    service: &str,
    session: Option<AdminSession>,
    return_to: ReturnTo,
    pool: &State<DbPool>,
) -> Result<Template, AdminReject>

The “register an existing service on a corpus” screen: a <select> over the corpora this service is not yet activated on (already-activated corpora are excluded — re-activating is destructive). Signed-in admins only (anonymous → sign-in). 404 if the service is unknown.