refactor(settings): 统一语言设置，合并 interface_language 和 ai_language

- 后端 Schema 将 interface_language 和 ai_language 合并为 language
- 前端设置界面只保留一个语言选项
- AI 回复语言统一使用 language 设置
- 更新协议文档
- 新增数据库迁移脚本

backend/tests/unit/test_parse_profile_settings.py

@@ -15,8 +15,7 @@ class TestParseProfileSettings:
         raw = {
             "version": 1,
             "preferences": {
-                "interface_language": "en-US",
-                "ai_language": "en-US",
+                "language": "en-US",
                 "timezone": "America/New_York",
                 "country": "US",
             },
@@ -31,8 +30,7 @@ class TestParseProfileSettings:
         assert isinstance(result, ProfileSettingsV1)
         assert result.version == 1
         assert isinstance(result.preferences, PreferenceSettings)
-        assert result.preferences.interface_language == "en-US"
-        assert result.preferences.ai_language == "en-US"
+        assert result.preferences.language == "en-US"
         assert result.preferences.timezone == "America/New_York"
         assert result.preferences.country == "US"
         assert isinstance(result.notification, NotificationSettings)
@@ -45,8 +43,7 @@ class TestParseProfileSettings:
         assert isinstance(result, ProfileSettingsV1)
         assert result.version == 1
         assert isinstance(result.preferences, PreferenceSettings)
-        assert result.preferences.interface_language == "zh-CN"
-        assert result.preferences.ai_language == "zh-CN"
+        assert result.preferences.language == "zh-CN"
         assert result.preferences.timezone == "Asia/Shanghai"
         assert result.preferences.country == "US"
         assert isinstance(result.notification, NotificationSettings)
@@ -56,13 +53,12 @@ class TestParseProfileSettings:
     def test_parse_profile_settings_with_partial_preferences(self) -> None:
         raw = {
             "preferences": {
-                "interface_language": "en-US",
+                "language": "en-US",
             },
         }
         result = parse_profile_settings(raw)
 
-        assert result.preferences.interface_language == "en-US"
-        assert result.preferences.ai_language == "zh-CN"
+        assert result.preferences.language == "en-US"
         assert result.preferences.timezone == "Asia/Shanghai"
         assert result.preferences.country == "US"
 
@@ -93,7 +89,7 @@ class TestParseProfileSettings:
     def test_parse_profile_settings_invalid_language_uses_default(self) -> None:
         raw = {
             "preferences": {
-                "interface_language": "invalid-language",
+                "language": "invalid-language",
             },
         }
         with pytest.raises(ValueError, match="language must be a valid BCP-47 tag"):
@@ -120,8 +116,7 @@ class TestParseProfileSettings:
     def test_profile_settings_v1_model_dump(self) -> None:
         settings = ProfileSettingsV1(
             preferences=PreferenceSettings(
-                interface_language="en-US",
-                ai_language="en-US",
+                language="en-US",
                 timezone="UTC",
                 country="US",
             ),
@@ -132,6 +127,6 @@ class TestParseProfileSettings:
         )
         dumped = settings.model_dump(mode="json")
 
-        assert dumped["preferences"]["interface_language"] == "en-US"
+        assert dumped["preferences"]["language"] == "en-US"
         assert dumped["notification"]["allow_notifications"] is True
         assert dumped["notification"]["allow_vibration"] is False