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/tasks/story_title_tasks.py

@@ -51,8 +51,19 @@ def generate_story_title_after_create(
                     ms,
                 )
                 return {"status": "skip_not_found"}
-            expected_ph = _placeholder_title(chapter_category)
-            if (st.title or "").strip() and (st.title or "").strip() != expected_ph:
+            user_obj_pre = db.get(User, user_id)
+            user_language = (
+                "en"
+                if user_obj_pre is not None
+                and str(getattr(user_obj_pre, "language_preference", "zh") or "zh").lower()
+                == "en"
+                else "zh"
+            )
+            expected_ph_zh = _placeholder_title(chapter_category, language="zh")
+            expected_ph_en = _placeholder_title(chapter_category, language="en")
+            expected_ph = _placeholder_title(chapter_category, language=user_language)
+            current = (st.title or "").strip()
+            if current and current not in (expected_ph_zh, expected_ph_en):
                 ms = (time.perf_counter() - t0) * 1000
                 logger.info(
                     "event=story_title_task_skip story_id={} reason=user_modified duration_ms={:.1f} "
@@ -73,7 +84,7 @@ def generate_story_title_after_create(
                 )
                 return {"status": "skip_no_llm"}
 
-            user_obj = db.get(User, user_id)
+            user_obj = user_obj_pre
             user_profile = ""
             birth_year = None
             if user_obj:
@@ -83,6 +94,7 @@ def generate_story_title_after_create(
                     birth_place=user_obj.birth_place,
                     grew_up_place=user_obj.grew_up_place,
                     occupation=user_obj.occupation,
+                    language=user_language,
                 )
 
             state = get_or_create_state_sync(user_id, db)
@@ -101,8 +113,13 @@ def generate_story_title_after_create(
                 user_birth_year=birth_year,
                 llm=llm,
                 oral_scope=oral_scope or "",
+                language=user_language,
             )
-            if not new_title.strip() or new_title.strip() == expected_ph:
+            if not new_title.strip() or new_title.strip() in (
+                expected_ph_zh,
+                expected_ph_en,
+                expected_ph,
+            ):
                 ms = (time.perf_counter() - t0) * 1000
                 logger.info(
                     "event=story_title_task_skip story_id={} reason=placeholder duration_ms={:.1f} "