fix(tts): gate auto reply by ENABLE_TTS; allow on-demand and manual playback

- Pipeline: skip _send_tts_audio only for non-manual when ENABLE_TTS=false;
  remove enable_tts early return from handle_tts_request_on_demand.
- Tencent TTS: PrimaryLanguage/chunking follow user language preference only.
- Expo: let manual tts_audio bypass late-segment playback gate after interrupt.
- Docs: clarify ENABLE_TTS vs tts_request in api/.env.example and TTSProvider port.
- Tests: add manual bypass cases; adjust pipeline language tests for en+Chinese text.

Co-authored-by: Cursor <cursoragent@cursor.com>

app-expo/src/app/(main)/conversation/[id].tsx

@@ -1238,7 +1238,10 @@ export default function ConversationScreen() {
 
   const handleTtsSegment = useCallback(
     (p: TtsSegmentPayload) => {
-      if (!ttsGate.current.shouldAcceptIncomingTts()) return;
+      // 闸门用于丢弃「用户已打断后」迟到的自动 TTS；按需朗读 (manual) 是当前明确操作，必须放行。
+      const allowByGate =
+        p.manual === true || ttsGate.current.shouldAcceptIncomingTts();
+      if (!allowByGate) return;
       const convId = id ?? '';
       const cosUrl = p.audioUrl?.trim();
       /**