wire pub/sub channel subscriptions into channel_focus handler

When a client focuses a channel, subscribe to its pub/sub topic.
When switching channels, unsubscribe from the old topic first.
This completes the pub/sub integration — channel broadcasts now
route only to clients subscribed to the relevant topic.

https://claude.ai/code/session_01CBFF3r84ywkJRWwuqw8zD8

Server/ws/export_test.go

@@ -94,9 +94,15 @@ func (p *LiveKitProcess) SetProcessStoppedForTest() {
 func NewHubForTest() *Hub {
 	return &Hub{
 		clients: make(map[int64]*Client),
+		pubsub:  NewPubSub(),
 	}
 }
 
+// PubSubForTest exposes the hub's PubSub for external tests.
+func (h *Hub) PubSubForTest() *PubSub {
+	return h.pubsub
+}
+
 // BuildAuthOKForTest exposes Hub.buildAuthOK for external tests.
 func (h *Hub) BuildAuthOKForTest(user *db.User, roleName string) []byte {
 	return h.buildAuthOK(user, roleName)

Server/ws/handlers.go

@@ -160,9 +160,20 @@ func (h *Hub) handleMessage(c *Client, raw []byte) {
 			}
 			// Apply client state mutations.
 			if result.SetChannelID != nil {
+				oldChID := c.getChannelID()
 				c.mu.Lock()
 				c.channelID = *result.SetChannelID
 				c.mu.Unlock()
+				// Update pub/sub channel topic subscriptions.
+				newChID := *result.SetChannelID
+				if oldChID != newChID {
+					if oldChID > 0 {
+						c.hub.pubsub.Unsubscribe(c, ChannelTopic(oldChID))
+					}
+					if newChID > 0 {
+						c.hub.pubsub.Subscribe(c, ChannelTopic(newChID))
+					}
+				}
 			}
 			if result.SetE2EEPubKey != nil {
 				c.setE2EEPubKey(*result.SetE2EEPubKey)