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/api/events_stream.go

@@ -105,7 +105,8 @@ func (h *handlers) handleEventsStream(w http.ResponseWriter, r *http.Request) {
 
 		case e, ok := <-ch:
 			if !ok {
-				// Bus closed our subscription (rare; only on full shutdown).
+				// The bus closed at shutdown. Ending here lets the
+				// server drain; the client reconnects to the next one.
 				return
 			}
 			// Scope filter: forward only events scoped to this user or