Function tick_once 

async fn tick_once(
    state: &Arc<AppState>,
    allow: &Arc<HashSet<String>>,
    last_run: &mut HashMap<MeasureId, u64>,
    start: Instant,
) -> Result<Vec<MeasureId>, String>

One scheduler tick: load the model, refresh due CALL measures, save if any were refreshed. Returns the ids refreshed. All I/O runs on a blocking thread, matching the request handlers’ spawn_blocking pattern.