feat: implement PATCH /user/settings/{option} and fix timezone range to UTC-12~+14

- Add SettingOption StrEnum (nickname/language/timezone) for path param validation
- Add UserSettingUpdateRequest DTO with Pydantic constraints
- Implement endpoint: extract value by option name, validate non-null for required fields
- Fix timezone upper bound from 12 to 14 (UTC+14 exists, e.g. Line Islands)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sqlmodels/__init__.py

@@ -14,6 +14,8 @@ from .user import (
     UserResponse,
     UserSettingResponse,
     UserThemeUpdateRequest,
+    SettingOption,
+    UserSettingUpdateRequest,
     WebAuthnInfo,
     UserTwoFactorResponse,
     # 管理员DTO