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>

Dockerfile

@@ -8,7 +8,7 @@ WORKDIR /app
 COPY package.json package-lock.json* ./
 RUN npm ci --omit=dev
 
-COPY server.js db.js ./
+COPY server.js db.js ai-prompt.txt ./
 COPY public ./public
 
 EXPOSE 3000