fix(sites): use the archive title when a bilibili dynamic has no body

A 视频投稿动态 (`DYNAMIC_TYPE_AV`) carries no body at all: the API
answers `desc: null` and the content is the archive card, so `title`
(and `{title}` in caption formats) stayed empty for the most common
dynamic type. Audited 24 live dynamics: every dynamic that *has* text
(a 图文 post, a forward, a text post) keeps it in
`module_dynamic.desc.text` — only the AV card has none, so the video
title now stands in, mirroring pixiv whose `title` is the artwork title
rather than post text.

Also records two API observations in comments/docs: an id that cannot
exist answers `4101105 请求数据发生错误` (kept on the permanent arm), and
the feed endpoints strip `desc.text` so only the detail endpoint shows
whether a post has text.

crates/x-media/src/site/bilibili/model.rs

@@ -97,6 +97,10 @@ pub(crate) struct Archive {
     /// video itself is deliberately not resolved, see the module docs).
     #[serde(default)]
     pub(crate) cover: Option<String>,
+    /// The video's title. An AV dynamic has no body of its own (`desc` comes
+    /// back `null`), so this card title is the post's content.
+    #[serde(default)]
+    pub(crate) title: Option<String>,
 }
 
 #[derive(Deserialize, Debug)]