feat(server): UDP listener, hub WebSocket, Gauges Prometheus

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/websocket/hub.go

@@ -0,0 +1,51 @@
+package websocket
+
+import (
+	"encoding/json"
+	"log"
+	"sync"
+
+	"github.com/gorilla/websocket"
+)
+
+type Hub struct {
+	mu      sync.RWMutex
+	clients map[*websocket.Conn]struct{}
+}
+
+func NewHub() *Hub {
+	return &Hub{clients: make(map[*websocket.Conn]struct{})}
+}
+
+func (h *Hub) Register(conn *websocket.Conn) {
+	h.mu.Lock()
+	h.clients[conn] = struct{}{}
+	h.mu.Unlock()
+}
+
+func (h *Hub) Unregister(conn *websocket.Conn) {
+	h.mu.Lock()
+	delete(h.clients, conn)
+	h.mu.Unlock()
+}
+
+func (h *Hub) Broadcast(msg interface{}) {
+	data, err := json.Marshal(msg)
+	if err != nil {
+		log.Printf("[ws] marshal: %v", err)
+		return
+	}
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	for conn := range h.clients {
+		if err := conn.WriteMessage(websocket.TextMessage, data); err != nil {
+			log.Printf("[ws] write: %v", err)
+		}
+	}
+}
+
+func (h *Hub) Count() int {
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	return len(h.clients)
+}