dashboard: filter /dashboard/stream by ?kinds= allow-list (#408)

hive-c0re/src/dashboard.rs

@@ -798,14 +798,49 @@ async fn dashboard_history(State(state): State<AppState>) -> Response {
     }
 }
 
+/// `/dashboard/stream` query string. Today's only field is `kinds`
+/// (#408): a comma-separated allow-list of event-`kind` strings.
+/// Empty / absent ⇒ no filter (current behaviour, all variants
+/// forwarded). Set ⇒ only the named kinds reach the subscriber,
+/// non-matches are skipped before the JSON serialise cost.
+///
+/// Useful for narrow pages (e.g. `flow.js` only cares about `sent`
+/// / `delivered` / `container_state_changed` / `container_removed`)
+/// that want to drop the dispatch overhead on every unrelated mutation.
+#[derive(Deserialize, Default)]
+struct DashboardStreamQuery {
+    /// Comma-separated event kinds to forward. Each token is
+    /// trimmed; unknown kinds are silently ignored on lookup
+    /// (subscriber sees nothing instead of an error).
+    kinds: Option<String>,
+}
+
 async fn dashboard_stream(
     State(state): State<AppState>,
+    axum::extract::Query(q): axum::extract::Query<DashboardStreamQuery>,
 ) -> Sse<impl Stream<Item = Result<Event, Infallible>>> {
     let rx = state.coord.dashboard_subscribe();
-    let stream = BroadcastStream::new(rx).filter_map(|res| {
+    // Pre-parse the allow-list once at subscription time, so the
+    // per-event hot path is just a `HashSet::contains` on a
+    // `&'static str` — no string churn per frame.
+    let kind_filter: Option<std::collections::HashSet<String>> = q.kinds.and_then(|raw| {
+        let set: std::collections::HashSet<String> = raw
+            .split(',')
+            .map(str::trim)
+            .filter(|s| !s.is_empty())
+            .map(str::to_owned)
+            .collect();
+        if set.is_empty() { None } else { Some(set) }
+    });
+    let stream = BroadcastStream::new(rx).filter_map(move |res| {
         // Drop lagged frames. Browsers reconnect; the seq dedupe on
         // reconnect skips any frame already reflected in the snapshot.
         let event = res.ok()?;
+        if let Some(filter) = kind_filter.as_ref()
+            && !filter.contains(event.kind_tag())
+        {
+            return None;
+        }
         let json = serde_json::to_string(&event).ok()?;
         Some(Ok(Event::default().data(json)))
     });