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/tests/test_pipeline_language_skip_tts.py

@@ -64,6 +64,29 @@ async def test_tencent_tts_zh_uses_primary_language_1_and_zh_voice() -> None:
     assert seen["voice_type"] == 501004
 
 
+@pytest.mark.asyncio
+async def test_tencent_tts_en_user_language_uses_primary_en_even_if_text_is_chinese() -> None:
+    """主语言与用户偏好一致：即使用户语言为 en 且正文为中文，也向 Tencent 提交 PrimaryLanguage=2。"""
+    provider = TencentTTSProvider(
+        secret_id="id",
+        secret_key="key",
+        voice_type=501004,
+        voice_type_en=501004,
+    )
+    seen: dict = {}
+
+    def fake_sync(text: str, voice_type: int, primary_language: int) -> bytes:
+        seen["primary_language"] = primary_language
+        seen["voice_type"] = voice_type
+        return b"OK"
+
+    with patch.object(provider, "_synthesize_sync", side_effect=fake_sync):
+        out = await provider.synthesize("这是中文回复。", language="en")
+
+    assert out == b"OK"
+    assert seen["primary_language"] == PRIMARY_LANGUAGE_EN
+
+
 @pytest.mark.asyncio
 async def test_tencent_tts_en_uses_primary_language_2_and_en_voice() -> None:
     provider = TencentTTSProvider(