feat(i18n): persist language preference and thread through chat, memoir, TTS

- Add users.language_preference (Alembic 0018, default zh); capture at signup/SMS
  only; expose on auth and profile APIs
- Lite English prompts for chat and memoir; localized stage labels and agent
  names (Life Echo / 岁月知己)
- Tencent TTS: language-aware synthesis, ModelType=1 for 501004, English chunking
- WebSocket pipeline: emit all AGENT_RESPONSE segments when TTS cancels; INFO logs
  for tts_this_turn and TTS decisions; on-demand TTS logging
- Expo: device language on auth, i18n tiers/agent name, [SPLIT] streaming UX fixes
- Tests for migration, prompts, pipeline, router tts_this_turn, reply segments

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

api/app/core/config.py

@@ -163,7 +163,13 @@ class Settings(BaseSettings):
     enable_tts: bool = True
     tts_provider: str = "tencent"
     openai_api_key: str = ""
-    tts_voice_type: int = 502001  # Tencent 音色 ID，见 https://cloud.tencent.com/document/product/1073/92668
+    # 501004 = 月华，腾讯云大模型音色，支持中英混合（PrimaryLanguage=1/2 均可）。
+    # 调用 TextToVoice 时必须配合 ModelType=1，详见 https://cloud.tencent.com/document/api/1073/37995
+    # 与音色清单 https://cloud.tencent.com/document/product/1073/92668
+    tts_voice_type: int = 501004
+    # 英文场景默认同样使用 501004（月华大模型音色，原生支持中英混合），
+    # 因此无需另配独立英文音色；如需切换英文专用音色请显式覆盖此项。
+    tts_voice_type_en: int = 501004
     tts_codec: str = "mp3"
 
     # ── WeChat Pay ───────────────────────────────────────────