feat(eventsource): add ConnectedUIDs accessor

Expose a snapshot of currently registered uids so fan-out broadcasters
can pre-filter recipients before calling SendMessage.

modules/eventsource/manager.go

@@ -68,6 +68,19 @@ func (m *Manager) UnregisterAll() {
 	m.messengers = map[int64]*Messenger{}
 }
 
+// ConnectedUIDs returns a snapshot of all currently registered user IDs.
+// Useful for fan-out broadcasters that need to filter recipients before
+// calling SendMessage.
+func (m *Manager) ConnectedUIDs() []int64 {
+	m.mutex.Lock()
+	defer m.mutex.Unlock()
+	uids := make([]int64, 0, len(m.messengers))
+	for uid := range m.messengers {
+		uids = append(uids, uid)
+	}
+	return uids
+}
+
 // SendMessage sends a message to a particular user
 func (m *Manager) SendMessage(uid int64, message *Event) {
 	m.mutex.Lock()