feat(call): enhance call handling with error differentiation and online notifications

- Added error handling for call invite and answer processes to differentiate between callee offline and call already answered scenarios.
- Introduced a new method to publish messages only to online users, improving real-time communication during call signaling.
- Updated call service to check callee's online status before sending call invites, ensuring better user experience.
- Enhanced call acceptance logic to notify both caller and callee about call status changes, including when a call is answered on another device.

internal/pkg/ws/hub.go

@@ -165,6 +165,41 @@ func (h *Hub) PublishToUsers(userIDs []string, eventType string, payload interfa
 	}
 }
 
+// PublishToUserOnline 仅在用户在线时发送事件，不存入历史回放
+// 适用于通话信令等实时性消息，避免断线重连时重放过期消息
+func (h *Hub) PublishToUserOnline(userID string, eventType string, payload interface{}) bool {
+	h.mu.RLock()
+	targets := make([]*Client, 0, len(h.clients[userID]))
+	for _, c := range h.clients[userID] {
+		targets = append(targets, c)
+	}
+	h.mu.RUnlock()
+
+	if len(targets) == 0 {
+		return false
+	}
+
+	msg := ResponseMessage{
+		EventID: h.NextID(),
+		Type:    eventType,
+		TS:      time.Now().UnixMilli(),
+		Payload: payload,
+	}
+	data, err := json.Marshal(msg)
+	if err != nil {
+		return false
+	}
+
+	for _, c := range targets {
+		select {
+		case <-c.Quit:
+		case c.Send <- data:
+		default:
+		}
+	}
+	return true
+}
+
 // publish 内部发布方法
 func (h *Hub) publish(userID string, ev Event) {
 	h.mu.Lock()