feat(setting): add move transfer threads option

[UVJkiNTQ]

Summary / 摘要

The move transfer task manager already read the move_task_threads_num setting, but the setting was never registered in the initial settings list, so move transfer concurrency was locked to its default (5) with no way to adjust it from the admin UI.

This PR registers move_task_threads_num in the Traffic settings group, mirroring the existing copy_task_threads_num entry. On startup initSettings() inserts the new key into the database, and the existing setting-change callback applies it to MoveTaskManager immediately without a restart.

用户可感知行为：管理后台 → 设置 → 传输(Traffic) 新增「移动任务线程数」(move_task_threads_num)，默认 5，保存后即时生效。

• This PR has breaking changes.
• This PR changes public API, config, storage format, or migration behavior.
• This PR requires corresponding changes in related repositories.

Testing / 测试

• Manual test / 手动测试：经 fork 的 Actions beta release 构建并部署后，move_task_threads_num 出现在 设置 → 传输 中，修改保存后生效。
• go test ./...：未运行（本环境无 Go 工具链）。本次改动仅为一行注册，与现有 copy_task_threads_num 写法一致。

Checklist / 检查清单

• I have read CONTRIBUTING.
• I confirm this contribution follows the repository license, contribution policy, and code of conduct.
• I have formatted the changed code with gofmt.
• I have requested review from relevant maintainers or code owners where applicable.

AI Disclosure / AI 使用声明

Minor AI assistance only: a one-line setting registration mirroring the adjacent existing entry, prepared with AI assistance. No Co-authored-by trailer is included; the submitting account is responsible for this contribution.

[PIKACHUIM]

@claude check if these changes are really taking effort to threads by move_task_threads_num