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/.env.example

@@ -247,7 +247,7 @@ TTS_PROVIDER=tencent
 # 仅 TTS_PROVIDER=openai 时需要
 # OPENAI_API_KEY=
 # 音色 ID 见 https://cloud.tencent.com/document/product/1073/92668
-TTS_VOICE_TYPE=502001
+TTS_VOICE_TYPE=501004
 TTS_CODEC=mp3
 
 # =============================================================================