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/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 });
     },
     [],