[Feature Request]: 人物/角色/章节的目录收藏

目录项目只支持 `Subject` 类， 对于人物、角色、章节都是错误数据。

目前针对这三个项目，都是拿其 ID 去 subject 库中找。

添加到目录，也不支持这三个分类

Bug2:
[/v0/indices/{index_id}/subjects](https://bangumi.github.io/api/#/%E7%9B%AE%E5%BD%95/getIndexSubjectsByIndexId) 中缺少对类别（Subject，人物，角色，章节）的过滤 （对应数据库 Cat 字段）

Start with the implementation and API documentation for /v0/indices/{index_id}/subjects, then trace the index-add and favorite paths mentioned in the issue. Check how the database Cat field represents Subject, character, person, and episode categories. Done means all four categories can be added to and retrieved from an index, with category filtering exposed by the endpoint.

Written by the indexing model from the issue text.