feat: add collaboration hub with room management

internal/collaboration/client.go

···
+package collaboration
+
+import (
+	"github.com/gorilla/websocket"
+)
+
+type Client struct {
+	hub     *Hub
+	conn    *websocket.Conn
+	send    chan []byte
+	DID     string
+	Name    string
+	Color   string
+	roomKey string
+}

internal/collaboration/hub.go

···
+package collaboration
+
+import (
+	"sync"
+)
+
+type Hub struct {
+	rooms map[string]*Room
+	mu    sync.RWMutex
+}
+
+type Room struct {
+	documentRKey string
+	clients      map[*Client]bool
+	broadcast    chan []byte
+	register     chan *Client
+	unregister   chan *Client
+	mu           sync.RWMutex
+}
+
+func NewHub() *Hub {
+	return &Hub{
+		rooms: make(map[string]*Room),
+	}
+}
+
+func (h *Hub) GetOrCreateRoom(rkey string) *Room {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	if room, exists := h.rooms[rkey]; exists {
+		return room
+	}
+	room := &Room{
+		documentRKey: rkey,
+		clients:      make(map[*Client]bool),
+		broadcast:    make(chan []byte, 256),
+		register:     make(chan *Client),
+		unregister:   make(chan *Client),
+	}
+	h.rooms[rkey] = room
+	go room.run()
+	return room
+}
+
+func (r *Room) run() {
+	for {
+		select {
+		case client := <-r.register:
+			r.mu.Lock()
+			r.clients[client] = true
+			r.mu.Unlock()
+			r.broadcastPresence()
+		case client := <-r.unregister:
+			r.mu.Lock()
+			if _, ok := r.clients[client]; ok {
+				delete(r.clients, client)
+				close(client.send)
+			}
+			r.mu.Unlock()
+			r.broadcastPresence()
+		case message := <-r.broadcast:
+			r.mu.RLock()
+			for client := range r.clients {
+				select {
+				case client.send <- message:
+				default:
+					close(client.send)
+					delete(r.clients, client)
+				}
+			}
+			r.mu.RUnlock()
+		}
+	}
+}
+
+func (r *Room) Broadcast(message []byte) {
+	r.broadcast <- message
+}
+
+func (r *Room) broadcastPresence() {
+	// Implementation in Task 2.2
+}