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>

.env.example

@@ -15,3 +15,13 @@ SESSION_SECRET=placeholder-session-secret-change-me
 # same engine as t2sTelegramBot). Used for newly created streamers; each
 # streamer's own voice is stored in the database and can be changed later.
 TTS_VOICE=ru-RU-DmitryNeural
+
+# Optional: DeepSeek API key (https://platform.deepseek.com) for the "AI reply"
+# checkbox on the viewer send page. If unset, the feature silently no-ops even
+# if a viewer checks the box. The system prompt used is in ai-prompt.txt.
+DEEPSEEK_API_KEY=
+
+# Voice used for AI replies specifically, deliberately different from a
+# streamer's own TTS_VOICE so listeners can tell an AI reply apart from a
+# human viewer's message.
+AI_VOICE=ru-RU-SvetlanaNeural