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/schemas.py

@@ -1,7 +1,9 @@
-from typing import Optional
+from typing import Literal, Optional
 
 from pydantic import BaseModel, Field
 
+LanguagePreference = Literal["zh", "en"]
+
 
 class RegisterRequest(BaseModel):
     phone: str = Field(..., min_length=11, max_length=11, description="手机号（11位）")
@@ -9,6 +11,10 @@ class RegisterRequest(BaseModel):
     nickname: str = Field(..., min_length=1, max_length=50, description="昵称")
     email: Optional[str] = Field(None, description="邮箱（可选）")
     agreed_to_terms: bool = Field(..., description="是否同意用户协议和隐私政策")
+    language: Optional[LanguagePreference] = Field(
+        None,
+        description="device language at signup; only used when creating a new user",
+    )
 
 
 class LoginRequest(BaseModel):
@@ -35,6 +41,7 @@ class UserResponse(BaseModel):
     avatar_url: Optional[str] = None
     subscription_type: str
     created_at: str
+    language_preference: LanguagePreference = "zh"
 
 
 class SendSmsRequest(BaseModel):
@@ -57,6 +64,10 @@ class SmsLoginRequest(BaseModel):
     nickname: Optional[str] = Field(
         None, max_length=50, description="昵称（注册时必填，登录时可选）"
     )
+    language: Optional[LanguagePreference] = Field(
+        None,
+        description="device language at signup; only used when creating a new user",
+    )
 
 
 class MockSmsLoginRequest(BaseModel):
@@ -67,6 +78,10 @@ class MockSmsLoginRequest(BaseModel):
     nickname: Optional[str] = Field(
         None, max_length=50, description="新用户昵称（可选）"
     )
+    language: Optional[LanguagePreference] = Field(
+        None,
+        description="device language at signup; only used when creating a new user",
+    )
 
 
 class SmsRegisterRequest(BaseModel):
@@ -76,6 +91,10 @@ class SmsRegisterRequest(BaseModel):
     nickname: str = Field(..., min_length=1, max_length=50, description="昵称")
     email: Optional[str] = Field(None, description="邮箱（可选）")
     agreed_to_terms: bool = Field(..., description="是否同意用户协议和隐私政策")
+    language: Optional[LanguagePreference] = Field(
+        None,
+        description="device language at signup; only used when creating a new user",
+    )
 
 
 class ResetPasswordRequest(BaseModel):