feat(interview): 支持面试问题按需中文翻译

[Scorbunny2]

功能描述

• 在沉浸式面试的面试官问题中，为“朗读”旁增加“翻译”按钮
• 点击后按需生成简体中文翻译，并在英文问题下方展示
• 再次点击可收起；同一页面再次展开复用按问题 ID 缓存的结果
• 翻译失败时允许重试，不产生对话消息、回答、轮数或评分记录

实现思路

• 新增只读接口 GET /v1/voice-questions/{question_id}/translation
• 服务端校验问题归属和会话类型，仅允许 INTERVIEW 场景
• 通过现有文本生成能力输出纯简体中文翻译，不写数据库
• Flutter 端仅在面试场景的助手问题上提供入口，并在页面内缓存结果

验证方式

• make check-api
• make check-go
• cd mobile && flutter test --no-pub（774 项通过）
• cd mobile && flutter test --no-pub test/practice/immersive_roleplay_test.dart
• cd mobile && flutter test --no-pub test/practice/wire_practice_client_test.dart test/agent/agent_message_bubble_test.dart

Closes #346

[fennoai]

No issues found in this review.

Verification:

• go test ./internal/practice/voice/... ./internal/bootstrap/... passed from server/.
• git diff --check 7078c7843aa45fa0578d46aa803157994457b806...ad7d91d489714eed7b76ced21c2c626240db2059 passed.
• Mobile Flutter tests were not run because flutter and dart are not installed in this sandbox.

Note: gh pr view reports mergeStateStatus: DIRTY, so this PR may need conflict resolution against dev before merge.