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_migration_language_preference_default.py

@@ -0,0 +1,34 @@
+"""验证 0018 迁移使用 server_default='zh' 落库（防止已有用户 NULL）。"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from app.features.user.models import User
+
+
+def test_migration_0018_uses_server_default_zh() -> None:
+    path = (
+        Path(__file__).resolve().parent.parent
+        / "alembic"
+        / "versions"
+        / "0018_users_language_preference.py"
+    )
+    src = path.read_text(encoding="utf-8")
+    assert "language_preference" in src
+    # server_default 'zh' 是已有用户回填的关键
+    assert "server_default=sa.text(\"'zh'\")" in src
+    assert "nullable=False" in src
+    # 在多行 op.add_column 调用中第一参数为 "users"
+    assert "op.add_column" in src
+    assert '"users"' in src
+
+
+def test_user_model_language_preference_default_zh() -> None:
+    """模型层默认值与迁移一致；新建实例不传值时为 'zh'。"""
+    column = User.__table__.c.language_preference
+    assert column.default is not None
+    assert column.default.arg == "zh"
+    assert column.server_default is not None
+    assert "zh" in str(column.server_default.arg)
+    assert column.nullable is False