add real-time infrastructure (#137)

* setup infra for pusher service

* setup client sdk for pusher service

* fix: ping parse failure

* fix: send pong back to client

avoid disconnections every 2.5 minutes

* increase replicas

* feat: show presence and compose indicator

go/internal/pusher/channel.go

@@ -0,0 +1,59 @@
+package pusher
+
+// Channel tracks the local connections subscribed to a channel on this pod.
+// All methods are only called from the Hub goroutine — no locks needed.
+type Channel struct {
+	id      string
+	members map[*Conn]string // conn → humanID
+}
+
+func newChannel(id string) *Channel {
+	return &Channel{
+		id:      id,
+		members: make(map[*Conn]string),
+	}
+}
+
+func (ch *Channel) addMember(conn *Conn, humanID string) {
+	ch.members[conn] = humanID
+}
+
+func (ch *Channel) removeMember(conn *Conn) {
+	delete(ch.members, conn)
+}
+
+func (ch *Channel) isEmpty() bool {
+	return len(ch.members) == 0
+}
+
+// localHumanIDs returns the deduplicated set of humanIDs connected on this pod.
+func (ch *Channel) localHumanIDs() []string {
+	seen := make(map[string]bool, len(ch.members))
+	ids := make([]string, 0, len(ch.members))
+	for _, hid := range ch.members {
+		if !seen[hid] {
+			seen[hid] = true
+			ids = append(ids, hid)
+		}
+	}
+	return ids
+}
+
+// hasHumanID returns true if the given humanID has at least one local connection.
+func (ch *Channel) hasHumanID(humanID string) bool {
+	for _, hid := range ch.members {
+		if hid == humanID {
+			return true
+		}
+	}
+	return false
+}
+
+// broadcast sends a message to all local connections except the excluded one.
+func (ch *Channel) broadcast(msg ServerMessage, exclude *Conn) {
+	for conn := range ch.members {
+		if conn != exclude {
+			conn.Send(msg)
+		}
+	}
+}