[Feature Request] /v0/indices/{index_id}/subjects 是否应该支持返回角色条目？

如题，Bangumi目录支持角色条目，但不支持通过API获取。

通过 `/v0/indices/{index_id}` 获得的目录信息中 `total` 属性是包括角色条目的，而 `/v0/indices/{index_id}/subjects` 却获取不到角色条目。

```js
// GET /v0/indices/75644
{
...
"total": 40,
"id": 75644,
...
}
```

```js
// GET /v0/indices/75644/subjects?limit=20&offset=0
{
"data": [],
"total": 0,
"limit": 20,
"offset": 0
}
```

Start by tracing the handlers for GET /v0/indices/{index_id} and /v0/indices/{index_id}/subjects, then inspect how index totals and subject results are assembled. The work is done when the subjects endpoint returns role entries supported by the index, with accurate total, limit, offset, and data fields.

Written by the indexing model from the issue text.