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/classification_agent.py

@@ -116,6 +116,7 @@ class ClassificationAgent:
         llm: Any,
         *,
         segment_id: str | None = None,
+        language: str = "zh",
     ) -> ChapterClassifyResult:
         """
         分类到 8 个章节类别之一。
@@ -138,7 +139,7 @@ class ClassificationAgent:
 
         if llm:
             try:
-                prompt = get_chapter_classification_json_prompt(text)
+                prompt = get_chapter_classification_json_prompt(text, language=language)
                 out = llm_json_call(
                     llm,
                     prompt,