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/agents/memoir/extraction_agent.py

@@ -35,6 +35,8 @@ class ExtractionAgent:
         current_stage: str,
         stage_slots: Dict[str, Any],
         llm: Any,
+        *,
+        language: str = "zh",
     ) -> ExtractionResult:
         """
         提取结构化信息并判断阶段。
@@ -56,6 +58,7 @@ class ExtractionAgent:
                     k: v.model_dump() if hasattr(v, "model_dump") else v
                     for k, v in (stage_slots or {}).items()
                 },
+                language=language,
             )
             parsed = llm_json_call(
                 llm,