Add SSE broadcaster

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/web/broadcaster_test.go

@@ -0,0 +1,42 @@
+package web
+
+import (
+	"testing"
+	"time"
+)
+
+func TestBroadcasterDeliversToSubscribers(t *testing.T) {
+	b := NewBroadcaster()
+	ch := b.Subscribe()
+	defer b.Unsubscribe(ch)
+
+	b.Publish("hello")
+
+	select {
+	case msg := <-ch:
+		if msg != "hello" {
+			t.Fatalf("got %q want hello", msg)
+		}
+	case <-time.After(time.Second):
+		t.Fatal("subscriber did not receive message")
+	}
+}
+
+func TestBroadcasterDropsWhenSubscriberFull(t *testing.T) {
+	b := NewBroadcaster()
+	ch := b.Subscribe()
+	defer b.Unsubscribe(ch)
+	// Publishing many messages without reading must not block (slow client).
+	done := make(chan struct{})
+	go func() {
+		for i := 0; i < 100; i++ {
+			b.Publish("x")
+		}
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("Publish blocked on a full subscriber")
+	}
+}