Function purge_cache 

async fn purge_cache(
    __arg0: State<Arc<SharedState>>,
    __arg1: Path<String>,
) -> impl IntoResponse

Purges the pair id on every backend that holds it.

404 when no compiled policy has a pair with that id: a typo must not read as a successful flush of nothing. 502 when a backend could not be reached, naming it and listing what did succeed first.