Live page: streamed updates and a sliding chart

The server pushes each new step over server-sent events
(GET /api/v1/live/stream) the moment it is sampled, so updates no
longer arrive in uneven pairs. The chart slides left steadily between
steps instead of jumping, and the big numbers count to their new
value. Both stay still with reduced motion.

api.go

@@ -146,6 +146,7 @@ func (a *App) routes() http.Handler {
 	g("GET /api/v1/status", a.status)
 	g("GET /api/v1/stats", a.allStats)
 	g("GET /api/v1/live", a.liveSpeeds)
+	g("GET /api/v1/live/stream", a.liveStream)
 
 	g("GET /api/v1/server", a.getServer)
 	g("PATCH /api/v1/server", a.patchServer)
@@ -412,6 +413,48 @@ func (a *App) liveSpeeds(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, map[string]any{"step": int(speedStep / time.Second), "size": speedPoints, "points": points})
 }
 
+// liveStream sends the same data as server-sent events: the current points
+// first, then each new step as soon as it is sampled. The session is checked
+// again with every step, so signing out ends the stream.
+func (a *App) liveStream(w http.ResponseWriter, r *http.Request) {
+	if a.speeds == nil {
+		writeJSON(w, http.StatusServiceUnavailable, map[string]string{"error": "live speeds are not available"})
+		return
+	}
+	rc := http.NewResponseController(w)
+	_ = rc.SetWriteDeadline(time.Time{}) // the server's write timeout would cut the stream
+	w.Header().Set("Content-Type", "text/event-stream")
+	w.Header().Set("Cache-Control", "no-store")
+	w.Header().Set("X-Accel-Buffering", "no") // nginx: do not buffer
+	points, ch, cancel := a.speeds.Subscribe()
+	defer cancel()
+	send := func(points []SpeedPoint) bool {
+		b, _ := json.Marshal(map[string]any{"step": int(speedStep / time.Second), "size": speedPoints, "points": points})
+		if _, err := fmt.Fprintf(w, "data: %s\n\n", b); err != nil {
+			return false
+		}
+		return rc.Flush() == nil
+	}
+	if !send(points) {
+		return
+	}
+	for {
+		select {
+		case <-r.Context().Done():
+			return
+		case <-a.speeds.Done():
+			return
+		case pt := <-ch:
+			if _, ok := a.auth.Authenticate(r); !ok {
+				return
+			}
+			if !send([]SpeedPoint{pt}) {
+				return
+			}
+		}
+	}
+}
+
 func (a *App) peerStats(w http.ResponseWriter, r *http.Request) {
 	cfg := a.store.Get()
 	if _, p := cfg.peerByID(r.PathValue("id")); p == nil {