Add optional AI reply via DeepSeek

Viewers can check a box on the send page to get a short AI-generated
reply to their message, voiced with a distinct TTS voice and queued
on the overlay after the original message. The system prompt lives
in ai-prompt.txt (editable without touching code) instead of being
hardcoded. TTS for both viewer messages and AI replies now announces
who it's from ("Сообщение от X." / "Ответ от ИИ.") before the text.
No-ops entirely if DEEPSEEK_API_KEY isn't set.

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

server.js

@@ -1,5 +1,6 @@
 require('dotenv').config();
 
+const fs = require('fs');
 const path = require('path');
 const express = require('express');
 const session = require('express-session');
@@ -21,6 +22,10 @@ const PORT = process.env.PORT || 3000;
 const GOOGLE_CLIENT_ID = process.env.GOOGLE_CLIENT_ID;
 const SESSION_SECRET = process.env.SESSION_SECRET;
 const DEFAULT_TTS_VOICE = process.env.TTS_VOICE || 'ru-RU-DmitryNeural';
+const DEEPSEEK_API_KEY = process.env.DEEPSEEK_API_KEY;
+const AI_VOICE = process.env.AI_VOICE || 'ru-RU-SvetlanaNeural';
+const AI_PROMPT_PATH = path.join(__dirname, 'ai-prompt.txt');
+const FALLBACK_AI_PROMPT = 'Ты дружелюбный ИИ-ассистент в чате стрима. Отвечай коротко и по делу.';
 
 if (!GOOGLE_CLIENT_ID) {
   console.error('Missing GOOGLE_CLIENT_ID in .env (see .env.example)');
@@ -177,6 +182,40 @@ function synthesizeSpeech(text, voice) {
   });
 }
 
+function getAiSystemPrompt() {
+  try {
+    const content = fs.readFileSync(AI_PROMPT_PATH, 'utf8').trim();
+    return content || FALLBACK_AI_PROMPT;
+  } catch {
+    return FALLBACK_AI_PROMPT;
+  }
+}
+
+async function getAiReply(text) {
+  const res = await fetch('https://api.deepseek.com/chat/completions', {
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+      Authorization: `Bearer ${DEEPSEEK_API_KEY}`,
+    },
+    body: JSON.stringify({
+      model: 'deepseek-chat',
+      messages: [
+        { role: 'system', content: getAiSystemPrompt() },
+        { role: 'user', content: text },
+      ],
+      max_tokens: 150,
+      temperature: 0.8,
+    }),
+  });
+
+  if (!res.ok) {
+    throw new Error(`DeepSeek API error: ${res.status} ${await res.text()}`);
+  }
+  const data = await res.json();
+  return data.choices[0].message.content.trim();
+}
+
 io.on('connection', (socket) => {
   const { role, token } = socket.handshake.query;
   const connectedUser = socket.request.session.user;
@@ -196,7 +235,7 @@ io.on('connection', (socket) => {
     console.log(`[socket] disconnected id=${socket.id} reason=${reason}`);
   });
 
-  socket.on('send_message', ({ token: senderToken, text, name } = {}) => {
+  socket.on('send_message', ({ token: senderToken, text, name, aiReply } = {}) => {
     const streamer = findStreamerBySenderToken(senderToken);
     if (!streamer) {
       socket.emit('send_error', 'Unknown link');
@@ -238,7 +277,7 @@ io.on('connection', (socket) => {
       // 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)
+      synthesizeSpeech(`Сообщение от ${displayName}. ${message}`, streamer.tts_voice || DEFAULT_TTS_VOICE)
         .then((audioBuffer) => {
           io.to(room).emit('display_audio', {
             id: messageId,
@@ -249,6 +288,36 @@ io.on('connection', (socket) => {
         .catch((err) => {
           console.error('[tts] synthesis failed:', err);
         });
+
+      if (aiReply && DEEPSEEK_API_KEY) {
+        getAiReply(message)
+          .then((replyText) => {
+            const aiMessageId = crypto.randomUUID();
+            console.log(`[ai] streamer=${streamer.id} id=${aiMessageId} reply="${replyText}"`);
+            io.to(room).emit('display_message', {
+              id: aiMessageId,
+              text: replyText,
+              from: '🤖 ИИ',
+              at: Date.now(),
+              durationMs: (streamer.display_duration_seconds || 10) * 1000,
+            });
+
+            synthesizeSpeech(`Ответ от ИИ. ${replyText}`, AI_VOICE)
+              .then((audioBuffer) => {
+                io.to(room).emit('display_audio', {
+                  id: aiMessageId,
+                  audio: audioBuffer.toString('base64'),
+                  mimeType: 'audio/mpeg',
+                });
+              })
+              .catch((err) => {
+                console.error('[tts] AI reply synthesis failed:', err);
+              });
+          })
+          .catch((err) => {
+            console.error('[ai] DeepSeek reply failed:', err);
+          });
+      }
     });
   });
 });