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/adapters/tts/tencent_tts.py

@@ -82,9 +82,8 @@ class TencentTTSProvider:
         self._secret_id = secret_id
         self._secret_key = secret_key
         self._voice_type = voice_type
-        # 英文音色未单独配置时回落到 501004（月华，腾讯云大模型音色，支持中英混合）。
-        # 大模型音色 501xxx 系列在 PrimaryLanguage=1/2 下均支持中英混读，不会被 Tencent
-        # 以 InvalidParameterValue.PrimaryLanguage 拒绝；与之对应必须配合 ModelType=1。
+        # 英文音色未单独配置时回落到 501004（月华，腾讯云大模型音色）。
+        # 大模型音色 501xxx 须配合 ModelType=1（见 Tencent TextToVoice 文档）。
         self._voice_type_en = voice_type_en if voice_type_en is not None else 501004
         self._codec = codec
         self._client = None
@@ -211,6 +210,8 @@ class TencentTTSProvider:
             )
             return b""
 
+        # ``language`` 由 pipeline 从用户 ``language_preference`` 解析（仅 'en' / 其它→中文路径），
+        # 与助手正文实际语种无关：产品规则是 TTS 主语言跟用户语言一致。
         is_en = (language or "zh").strip().lower() == "en"
         primary_language = PRIMARY_LANGUAGE_EN if is_en else PRIMARY_LANGUAGE_ZH
         default_voice = self._voice_type_en if is_en else self._voice_type