Introduce model/ and repository/ structure under lib/

Models: Message, WikiPage, Game (typed structs instead of raw hashes)
Repositories: MessageBoard, OnlineUsers, WikiClient, CatalogClient
bbs.rb uses attribute access (page.title, game.play_path, …) throughout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/repository/online_users.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class OnlineUsers
+  def initialize
+    @users = {}
+    @mu    = Mutex.new
+  end
+
+  def add(session_id, name)
+    @mu.synchronize { @users[session_id] = name }
+  end
+
+  def remove(session_id)
+    @mu.synchronize { @users.delete(session_id) }
+  end
+
+  def snapshot
+    @mu.synchronize { @users.dup }
+  end
+
+  def count
+    @mu.synchronize { @users.size }
+  end
+end