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/router.py

@@ -17,7 +17,7 @@ from app.features.user.schemas import (
     UpdateUserProfileRequest,
     UserProfileResponse,
 )
-from app.features.user.service import UserService
+from app.features.user.service import UserService, _coerce_language as _coerce_language_token
 from app.ports.storage import ObjectStorage
 
 logger = get_logger(__name__)
@@ -57,6 +57,9 @@ async def get_user_profile(
         birth_place=current_user.birth_place,
         grew_up_place=current_user.grew_up_place,
         occupation=current_user.occupation,
+        language_preference=_coerce_language_token(
+            getattr(current_user, "language_preference", "zh")
+        ),
     )