Queue overlay messages instead of interrupting the current one

Messages now play one at a time for their full configured duration
before the next one appears, instead of a new message immediately
replacing whatever was on screen. TTS audio is tagged with the same
message id as its text so it stays in sync even though it's
generated and delivered asynchronously after the text broadcast.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server.js

@@ -7,6 +7,7 @@ const { createServer } = require('http');
 const { Server } = require('socket.io');
 const { OAuth2Client } = require('google-auth-library');
 const { execFile } = require('child_process');
+const crypto = require('crypto');
 const {
   findStreamerByOverlayToken,
   findStreamerBySenderToken,
@@ -222,9 +223,11 @@ io.on('connection', (socket) => {
       const displayName = (typeof name === 'string' && name.trim().slice(0, 60)) || user.name;
       setViewerName(streamer.id, user.id, displayName);
 
+      const messageId = crypto.randomUUID();
       const room = `streamer-${streamer.id}`;
-      console.log(`[broadcast] streamer=${streamer.id} from="${displayName}" "${message}"`);
+      console.log(`[broadcast] streamer=${streamer.id} id=${messageId} from="${displayName}" "${message}"`);
       io.to(room).emit('display_message', {
+        id: messageId,
         text: message,
         from: displayName,
         at: Date.now(),
@@ -232,10 +235,13 @@ io.on('connection', (socket) => {
       });
 
       // Voice is generated after the fact so the text shows up immediately
-      // instead of waiting on the round trip to the TTS service.
+      // instead of waiting on the round trip to the TTS service. The overlay
+      // queues messages and matches this to the right one by id, since it may
+      // arrive before that message's turn to display.
       synthesizeSpeech(message, streamer.tts_voice || DEFAULT_TTS_VOICE)
         .then((audioBuffer) => {
           io.to(room).emit('display_audio', {
+            id: messageId,
             audio: audioBuffer.toString('base64'),
             mimeType: 'audio/mpeg',
           });