POST /v0/search/subjects 里用 - 排除标签没有效果

如题
`
POST /v0/search/subjects
{
"keyword": "",
"filter": {
"meta_tags": [
"-科幻"
]
}
}
`
会返回
`
{
"data": [],
"total": 0,
"limit": 10,
"offset": 0
}
`

Start by reproducing the POST /v0/search/subjects request shown in the issue, then trace the endpoint's handling of the meta_tags filter. Compare the -科幻 case with an ordinary tag filter. Done means the exclusion filter omits 科幻 subjects without incorrectly returning an empty result set.

Written by the indexing model from the issue text.