fix: shutdown ends the live-event streams so a redeploy drains instead of waiting out its deadline (#5293)

http.Server.Shutdown waits for in-flight requests but never cancels them,
and an SSE stream only ends when its client leaves, so any open tab or
signed-in phone held every redeploy for the full 10s and the old task
exited 1. With the swarm's stop-first update that was ~10s of downtime per
deploy. The bus now closes on shutdown, ending each stream; clients
reconnect to the next instance.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/eventbus/eventbus_test.go

@@ -148,3 +148,44 @@ func TestPublish_ConcurrentWritersAndSubscribers(t *testing.T) {
 	}
 	subWG.Wait()
 }
+
+// Close is what lets the server shut down with SSE clients connected: each
+// stream returns when its channel closes (#5293).
+func TestClose_EndsEverySubscription(t *testing.T) {
+	t.Parallel()
+	b := New()
+	ch1, unsub1 := b.Subscribe(4)
+	ch2, unsub2 := b.Subscribe(4)
+
+	b.Close()
+
+	for i, ch := range []<-chan Event{ch1, ch2} {
+		if _, ok := <-ch; ok {
+			t.Fatalf("subscriber %d: channel still open after Close", i)
+		}
+	}
+	if got := b.SubscriberCount(); got != 0 {
+		t.Fatalf("SubscriberCount=%d after Close, want 0", got)
+	}
+	// The handlers' deferred unsubscribes still run; they must not
+	// double-close.
+	unsub1()
+	unsub2()
+	b.Close()
+}
+
+func TestSubscribe_AfterCloseIsAlreadyClosed(t *testing.T) {
+	t.Parallel()
+	b := New()
+	b.Close()
+
+	ch, unsub := b.Subscribe(4)
+	defer unsub()
+	if _, ok := <-ch; ok {
+		t.Fatal("a subscription made after Close should already be closed")
+	}
+	if got := b.SubscriberCount(); got != 0 {
+		t.Fatalf("SubscriberCount=%d, want 0", got)
+	}
+	b.Publish(Event{Kind: "after.close"}) // must not panic
+}