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>

public/send.html

@@ -71,6 +71,18 @@
     color: #888;
     min-height: 20px;
   }
+  .checkbox-row {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    margin-top: 10px;
+    font-size: 14px;
+  }
+  .checkbox-row label {
+    display: inline;
+    margin-bottom: 0;
+    color: inherit;
+  }
   #hidden-until-login {
     display: none;
   }
@@ -94,7 +106,12 @@
   <input id="display-name" type="text" maxlength="60" placeholder="Имя..." />
 
   <textarea id="message" placeholder="Текст для стрима..." maxlength="500"></textarea>
-  <br />
+
+  <div class="checkbox-row">
+    <input id="ai-reply" type="checkbox" />
+    <label for="ai-reply">Получить ответ от ИИ 🤖</label>
+  </div>
+
   <button id="send">Отправить</button>
   <div id="status"></div>
 </div>