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>

app-expo/src/features/auth/hooks.ts

@@ -5,9 +5,11 @@ import { useCallback } from 'react';
 import { AuthError } from '@/core/api/types';
 import { tokenManager } from '@/core/auth/token-manager';
 import { disposeAllBackgroundConversationWs } from '@/features/conversation/conversation-ws-background-pool';
+import { getDeviceLanguage } from '@/i18n';
 
 import { authApi } from './api';
 import type {
+  LanguagePreference,
   LoginRequest,
   RegisterRequest,
   SessionState,
@@ -19,6 +21,17 @@ import type {
   UserInfo,
 } from './types';
 
+/**
+ * Resolve the device language to send at sign-up. Backend only persists
+ * language_preference on first user creation; subsequent logins ignore it.
+ */
+function withDeviceLanguage<T extends { language?: LanguagePreference }>(
+  body: T,
+): T {
+  if (body.language) return body;
+  return { ...body, language: getDeviceLanguage() };
+}
+
 // ─── Query keys ───
 
 export const authKeys = {
@@ -139,7 +152,8 @@ export function useSmsLogin() {
   const onSuccess = usePostAuthSetup();
 
   return useMutation({
-    mutationFn: (body: SmsLoginRequest) => authApi.loginWithSms(body),
+    mutationFn: (body: SmsLoginRequest) =>
+      authApi.loginWithSms(withDeviceLanguage(body)),
     onSuccess,
   });
 }
@@ -150,7 +164,8 @@ export function useRegister() {
   const onSuccess = usePostAuthSetup();
 
   return useMutation({
-    mutationFn: (body: RegisterRequest) => authApi.register(body),
+    mutationFn: (body: RegisterRequest) =>
+      authApi.register(withDeviceLanguage(body)),
     onSuccess,
   });
 }
@@ -161,7 +176,8 @@ export function useSmsRegister() {
   const onSuccess = usePostAuthSetup();
 
   return useMutation({
-    mutationFn: (body: SmsRegisterRequest) => authApi.registerWithSms(body),
+    mutationFn: (body: SmsRegisterRequest) =>
+      authApi.registerWithSms(withDeviceLanguage(body)),
     onSuccess,
   });
 }

app-expo/src/features/auth/types.ts

@@ -1,5 +1,7 @@
 // ─── Response types ───
 
+export type LanguagePreference = 'zh' | 'en';
+
 export interface TokenResponse {
   access_token: string;
   refresh_token: string;
@@ -14,6 +16,7 @@ export interface UserInfo {
   avatar_url: string | null;
   subscription_type: string;
   created_at: string;
+  language_preference?: LanguagePreference;
 }
 
 // ─── Request types ───
@@ -30,6 +33,7 @@ export interface RegisterRequest {
   nickname: string;
   email?: string;
   agreed_to_terms: boolean;
+  language?: LanguagePreference;
 }
 
 export type SmsPurpose =
@@ -48,6 +52,7 @@ export interface SmsLoginRequest {
   code: string;
   agreed_to_terms: boolean;
   nickname?: string;
+  language?: LanguagePreference;
 }
 
 export interface SmsRegisterRequest {
@@ -57,6 +62,7 @@ export interface SmsRegisterRequest {
   nickname: string;
   email?: string;
   agreed_to_terms: boolean;
+  language?: LanguagePreference;
 }
 
 export interface ResetPasswordRequest {

app-expo/src/features/conversation/hooks.ts

@@ -3,6 +3,7 @@ import { File, Paths } from 'expo-file-system';
 import { useCallback, useEffect, useRef, useState } from 'react';
 import { AppState, type AppStateStatus } from 'react-native';
 
+import i18n from '@/i18n';
 import type { WsConnectionState } from '@/core/ws/types';
 
 import { conversationApi } from './api';
@@ -111,7 +112,7 @@ export function useCreateConversation() {
           const now = Date.now();
           const item: ConversationListItem = {
             id: newConversation.id,
-            title: '岁月知己',
+            title: i18n.t('agentName', { ns: 'conversation' }),
             avatarUrl: null,
             latestMessagePreview: '',
             latestMessageTime: now,
@@ -247,6 +248,14 @@ export function useRealtimeSession({
         setAwaitingAssistantReply(false);
         return;
       }
+      /**
+       * 空文本 + 未完成时不能写成 `{text: '', isComplete: false}`：
+       * UI 会渲染一只空 `StreamingBubbles`（pulsing 气泡 + 光标），看上去与
+       * 「正在回复…」typing 气泡难以区分，且会一直挂在底部不消失。
+       */
+      if (text.length === 0) {
+        return;
+      }
       setStreamingMessage({ text, isComplete });
     },
     [],