Add graceful shutdown for SSE connections

Add atomic shutdown flag and shutdown channel to handle graceful
termination of Server-Sent Events connections. This ensures:

1. New connections are rejected with a proper status code during shutdown
2. Existing connections receive a notification before being closed
3. All subscriber channels are properly unsubscribed from the broker

This implementation safely handles multiple in-flight requests during
shutdown by using an atomic flag to track server state.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

server/server.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"net/http"
 	"strconv"
+	"sync/atomic"
 	"time"
 
 	"github.com/dpup/prefab"
@@ -24,6 +25,10 @@ type Config struct {
 type Server struct {
 	config Config
 	server *prefab.Server
+	// Channel to signal shutdown to active connections
+	shutdown chan struct{}
+	// Atomic flag to indicate if server is shutting down
+	isShuttingDown atomic.Bool
 }
 
 // New creates a new server instance
@@ -33,7 +38,8 @@ func New(config Config) *Server {
 	}
 	
 	return &Server{
-		config: config,
+		config:   config,
+		shutdown: make(chan struct{}),
 	}
 }
 
@@ -61,6 +67,13 @@ func (s *Server) Start() error {
 
 // Stop shuts down the server
 func (s *Server) Stop() error {
+	// Set the shutdown flag first to prevent new connections from starting streams
+	s.isShuttingDown.Store(true)
+	
+	// Signal all active connections to close
+	close(s.shutdown)
+	
+	// Then shut down the HTTP server
 	if s.server != nil {
 		return s.server.Shutdown()
 	}
@@ -80,6 +93,12 @@ func (s *Server) handleStatus(w http.ResponseWriter, r *http.Request) {
 
 // handleStream handles Server-Sent Events streaming of MQTT messages
 func (s *Server) handleStream(w http.ResponseWriter, r *http.Request) {
+	// Check if the server is shutting down
+	if s.isShuttingDown.Load() {
+		http.Error(w, "Server is shutting down", http.StatusServiceUnavailable)
+		return
+	}
+	
 	// Check if broker is available
 	if s.config.Broker == nil {
 		http.Error(w, "MQTT broker not available", http.StatusServiceUnavailable)
@@ -118,6 +137,14 @@ func (s *Server) handleStream(w http.ResponseWriter, r *http.Request) {
 			s.config.Broker.Unsubscribe(packetChan)
 			return
 			
+		case <-s.shutdown:
+			// Server is shutting down, send a message to client and close
+			log.Println("Server shutting down, closing SSE connection")
+			fmt.Fprintf(w, "event: info\ndata: Server shutting down, connection closed\n\n")
+			flusher.Flush()
+			s.config.Broker.Unsubscribe(packetChan)
+			return
+			
 		case packet, ok := <-packetChan:
 			if !ok {
 				// Channel closed, probably shutting down