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/features/user/models.py

@@ -21,6 +21,9 @@ class User(Base):
     birth_place = Column(String, nullable=True)
     grew_up_place = Column(String, nullable=True)
     occupation = Column(String, nullable=True)
+    language_preference = Column(
+        String(8), nullable=False, default="zh", server_default="zh"
+    )
 
     conversations = relationship("Conversation", back_populates="user")
     chapters = relationship("Chapter", back_populates="user")