feat: add YouTube live poll support

Add backend support for YouTube live polls including:
- Poll event handling and state tracking
- New `/poll/current` endpoint for current poll data
- Server-sent events stream at `/poll/stream` for real-time updates
- Poll emitter for broadcasting poll changes to clients

shared/chat.ts

@@ -19,6 +19,11 @@ export type MessageRun = {
   emojiAlt?: string;
 };
 
+export type Poll = {
+  id: string; // live_chat_poll_id from YouTube
+  active: boolean;
+};
+
 export type ChatMessage = {
   id: string;
   author: string;