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/memoir_quality_pass_tasks.py

@@ -41,13 +41,16 @@ def _polish_story_title(
     llm,
     *,
     chapter_category: str,
+    language: str = "zh",
 ) -> bool:
     """Re-generate title if current title is a placeholder. Returns True if updated."""
     from app.features.memoir.story_pipeline_sync import _placeholder_title
 
     current = (story.title or "").strip()
-    placeholder = _placeholder_title(chapter_category)
-    if current and current != placeholder:
+    placeholder_zh = _placeholder_title(chapter_category, language="zh")
+    placeholder_en = _placeholder_title(chapter_category, language="en")
+    placeholder = _placeholder_title(chapter_category, language=language)
+    if current and current not in (placeholder_zh, placeholder_en):
         return False
 
     body = (story.canonical_markdown or "").strip()
@@ -63,9 +66,10 @@ def _polish_story_title(
         user_profile="",
         birth_year=None,
         llm=llm,
+        language=language,
     )
     new_title = (new_title or "").strip()
-    if not new_title or new_title == placeholder:
+    if not new_title or new_title in (placeholder_zh, placeholder_en, placeholder):
         return False
 
     story.title = new_title
@@ -138,6 +142,16 @@ def memoir_quality_pass(
         chapters_dirtied: set[str] = set()
 
         with get_sync_db() as db:
+            from app.features.user.models import User
+
+            user_obj = db.get(User, user_id)
+            user_language = (
+                "en"
+                if user_obj is not None
+                and str(getattr(user_obj, "language_preference", "zh") or "zh").lower()
+                == "en"
+                else "zh"
+            )
             for sid in story_ids:
                 story = db.get(Story, sid)
                 if not story or story.user_id != user_id:
@@ -145,7 +159,11 @@ def memoir_quality_pass(
 
                 chapter_category = story.stage or "summary"
                 if _polish_story_title(
-                    db, story, llm, chapter_category=chapter_category
+                    db,
+                    story,
+                    llm,
+                    chapter_category=chapter_category,
+                    language=user_language,
                 ):
                     titles_polished += 1
                     stmt = select(Chapter.id).where(