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/features/auth/service.py

@@ -24,6 +24,16 @@ CODE_LENGTH = 6
 CODE_EXPIRE_MINUTES = 5
 RATE_LIMIT_SECONDS = 60
 
+_VALID_LANGUAGES = {"zh", "en"}
+
+
+def _normalize_language(lang: str | None) -> str:
+    """Normalize device language token; default to zh on missing/unknown."""
+    if not lang:
+        return "zh"
+    s = str(lang).strip().lower()
+    return s if s in _VALID_LANGUAGES else "zh"
+
 
 class AuthError(Exception):
     def __init__(self, message: str, code: str = "AUTH_ERROR"):
@@ -120,6 +130,7 @@ class AuthService:
         password: str,
         nickname: str,
         email: str | None = None,
+        language: str | None = None,
     ) -> dict:
         """Register new user. Returns {user, access_token, refresh_token}."""
         if await repo.get_user_by_phone(phone, self._db):
@@ -137,6 +148,7 @@ class AuthService:
             nickname=nickname,
             subscription_type="free",
             created_at=datetime.now(timezone.utc),
+            language_preference=_normalize_language(language),
         )
         await repo.create_user(user, self._db)
         tokens = await self._issue_tokens(user_id)
@@ -206,6 +218,7 @@ class AuthService:
         code: str,
         device_info: str = "",
         nickname: str | None = None,
+        language: str | None = None,
     ) -> dict:
         """SMS login (auto-register if new). Returns {user, access_token, refresh_token, is_new_user}."""
         success = False
@@ -219,7 +232,10 @@ class AuthService:
             raise AuthError(message, "INVALID_SMS_CODE")
 
         return await self._sms_login_after_code_verified(
-            phone, device_info=device_info, nickname=nickname
+            phone,
+            device_info=device_info,
+            nickname=nickname,
+            language=language,
         )
 
     async def _sms_login_after_code_verified(
@@ -228,8 +244,12 @@ class AuthService:
         *,
         device_info: str = "",
         nickname: str | None = None,
+        language: str | None = None,
     ) -> dict:
-        """SMS 已校验通过后：查找或创建用户并签发令牌。"""
+        """SMS 已校验通过后：查找或创建用户并签发令牌。
+
+        ``language`` 仅在「新用户」分支下写入；命中已有用户时不覆盖偏好。
+        """
         user = await repo.get_user_by_phone(phone, self._db)
         is_new_user = user is None
 
@@ -242,6 +262,7 @@ class AuthService:
                 nickname=(nickname or "").strip(),
                 subscription_type="free",
                 created_at=datetime.now(timezone.utc),
+                language_preference=_normalize_language(language),
             )
             await repo.create_user(user, self._db)
 
@@ -257,10 +278,14 @@ class AuthService:
         phone: str,
         device_info: str = "",
         nickname: str | None = None,
+        language: str | None = None,
     ) -> dict:
         """跳过短信校验的登录/自动注册（仅由 mock 路由在配置允许时调用）。"""
         return await self._sms_login_after_code_verified(
-            phone, device_info=device_info, nickname=nickname
+            phone,
+            device_info=device_info,
+            nickname=nickname,
+            language=language,
         )
 
     async def register_with_sms(
@@ -271,6 +296,7 @@ class AuthService:
         nickname: str,
         email: str | None = None,
         device_info: str = "",
+        language: str | None = None,
     ) -> dict:
         """SMS register. Returns {user, access_token, refresh_token}."""
         success, message = await self._verify_sms_code(phone, code, "register")
@@ -292,6 +318,7 @@ class AuthService:
             nickname=nickname,
             subscription_type="free",
             created_at=datetime.now(timezone.utc),
+            language_preference=_normalize_language(language),
         )
         await repo.create_user(user, self._db)
         tokens = await self._issue_tokens(user_id, device_info)