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>

api/app/ports/tts.py

@@ -10,6 +10,6 @@ class TTSProvider(Protocol):
     ) -> bytes:
         """Convert text to speech audio bytes.
 
-        language: 'zh' or 'en'. Adapters that natively detect language may ignore it.
+        language: 'zh' or 'en' — 调用方应使用用户语言偏好（与正文语种无关）；各 adapter 按自身能力解释。
         """
         ...