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/tests/test_ws_router_tts_this_turn_passthrough.py

@@ -0,0 +1,165 @@
+"""路由器 → pipeline 的 ``tts_this_turn`` 字段传递契约测试。
+
+回归保护：FE（``app-expo/src/core/ws/client.ts`` 与
+``app-expo/src/features/conversation/realtime-session.ts``）一律以蛇形 ``tts_this_turn``
+作为 WebSocket payload 的 ``data`` 字段名；后端 ``ws/router.py`` 三个对话入口分支也按
+``data.get("tts_this_turn")`` 取值并传递给 pipeline。
+
+这里不依赖完整 WebSocket runtime，只在两处验证契约：
+
+1. **数据形状契约**：用 FE 实际发送的 payload 形状（snake_case key）调用
+   ``data.get("tts_this_turn")``，断言能取到 True；同时驼峰 ``ttsThisTurn`` 应取不到，
+   防止后续有人把后端 key 改成驼峰。
+2. **wrapper 透传契约**：直接驱动 ``process_persisted_user_segment_response``
+   （路由 TEXT / AUDIO_MESSAGE 分支唯一会调用的 pipeline 包装），mock 内部 DB +
+   ``process_user_message``，断言 ``tts_this_turn=True`` 被原样传到 ``process_user_message``。
+"""
+
+from __future__ import annotations
+
+from contextlib import asynccontextmanager
+from types import SimpleNamespace
+from unittest.mock import AsyncMock
+
+import pytest
+
+from app.features.conversation.ws import pipeline as ws_pipeline
+
+
+# ── 数据形状契约 ────────────────────────────────────────────────
+
+
+def test_router_text_payload_extracts_tts_this_turn_snake_case() -> None:
+    """FE 发送 ``{type:'text', data:{text, tts_this_turn:true}}``，后端按蛇形读取。"""
+    fe_payload_data = {"text": "hi", "tts_this_turn": True}
+    assert bool(fe_payload_data.get("tts_this_turn")) is True
+
+    # 缺省 / 显式 false：均应取到 False
+    assert bool({"text": "hi"}.get("tts_this_turn")) is False
+    assert bool({"text": "hi", "tts_this_turn": False}.get("tts_this_turn")) is False
+
+    # 驼峰回归保护：FE 历史曾误传过驼峰，蛇形 get 必须取不到，避免静默吞掉
+    assert bool({"text": "hi", "ttsThisTurn": True}.get("tts_this_turn")) is False
+
+
+def test_router_audio_segment_payload_extracts_tts_this_turn_snake_case() -> None:
+    """FE ``sendAudioSegment`` 也按 ``data.tts_this_turn`` 输出，蛇形保持一致。"""
+    fe_payload_data = {
+        "audio_base64": "AAA",
+        "segment_index": 0,
+        "voice_session_id": "vs-1",
+        "is_last": True,
+        "duration": 3,
+        "tts_this_turn": True,
+    }
+    assert bool(fe_payload_data.get("tts_this_turn")) is True
+
+
+# ── wrapper 透传契约 ────────────────────────────────────────────
+
+
+@pytest.mark.asyncio
+async def test_process_persisted_user_segment_response_passes_tts_this_turn(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """路由 TEXT 分支 → ``process_persisted_user_segment_response`` → ``process_user_message``。"""
+    captured: dict = {}
+
+    async def _spy_process_user_message(*args, **kwargs) -> None:
+        captured.update(kwargs)
+        # 兼容位置参数（实际调用是关键字调用，这里只为防御性记录）
+        captured["_args"] = args
+
+    monkeypatch.setattr(ws_pipeline, "process_user_message", _spy_process_user_message)
+
+    conversation = SimpleNamespace(
+        id="conv-1",
+        user_id="user-1",
+        deleted_at=None,
+        last_message_at=None,
+    )
+    user = SimpleNamespace(id="user-1", language_preference="zh")
+    segment = SimpleNamespace(
+        id="seg-1",
+        conversation_id="conv-1",
+        user_input_text="说说童年",
+        created_at=None,
+    )
+
+    fake_db = SimpleNamespace(
+        get=AsyncMock(
+            side_effect=lambda model, oid: {
+                "conv-1": conversation,
+                "user-1": user,
+                "seg-1": segment,
+            }.get(oid)
+        )
+    )
+
+    @asynccontextmanager
+    async def _fake_session_local():
+        yield fake_db
+
+    monkeypatch.setattr(ws_pipeline, "AsyncSessionLocal", _fake_session_local)
+
+    await ws_pipeline.process_persisted_user_segment_response(
+        conversation_id="conv-1",
+        user_id="user-1",
+        segment_id="seg-1",
+        tts_this_turn=True,
+    )
+
+    assert captured.get("tts_this_turn") is True, (
+        "router TEXT/AUDIO_MESSAGE 分支必须把 tts_this_turn=True 透传到 process_user_message"
+    )
+    assert captured.get("conversation_id") == "conv-1"
+    assert captured.get("user_message") == "说说童年"
+
+
+@pytest.mark.asyncio
+async def test_process_persisted_user_segment_response_default_tts_this_turn_false(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """缺省（未开本轮朗读）必须以 False 透传，不能被默默改成 True。"""
+    captured: dict = {}
+
+    async def _spy_process_user_message(*_args, **kwargs) -> None:
+        captured.update(kwargs)
+
+    monkeypatch.setattr(ws_pipeline, "process_user_message", _spy_process_user_message)
+
+    conversation = SimpleNamespace(
+        id="conv-2", user_id="user-2", deleted_at=None, last_message_at=None
+    )
+    user = SimpleNamespace(id="user-2", language_preference="zh")
+    segment = SimpleNamespace(
+        id="seg-2",
+        conversation_id="conv-2",
+        user_input_text="x",
+        created_at=None,
+    )
+
+    fake_db = SimpleNamespace(
+        get=AsyncMock(
+            side_effect=lambda model, oid: {
+                "conv-2": conversation,
+                "user-2": user,
+                "seg-2": segment,
+            }.get(oid)
+        )
+    )
+
+    @asynccontextmanager
+    async def _fake_session_local():
+        yield fake_db
+
+    monkeypatch.setattr(ws_pipeline, "AsyncSessionLocal", _fake_session_local)
+
+    await ws_pipeline.process_persisted_user_segment_response(
+        conversation_id="conv-2",
+        user_id="user-2",
+        segment_id="seg-2",
+        # 不传 tts_this_turn → 默认 False
+    )
+
+    assert captured.get("tts_this_turn") is False