refactor(x-media): split the post title from its content

`Fetched.title` carried whatever text the platform had — a tweet's body,
a bilibili dynamic's body, a pixiv artwork's title — which was enough
while x/twitter (no title at all) set the shape. The platforms actually
disagree: pixiv has a title *and* a description, bilibili has an opus
headline *and* a body. Posts now carry both:

- `title`: the platform's title (a pixiv artwork title, a bilibili opus
  headline or video card title), empty on text-only platforms;
- `content`: the body (tweet / bsky / misskey text, bilibili dynamic
  body, and pixiv's description — fetched for the first time here and
  flattened from the app API's HTML to plain text).

`{content}` joins the caption-format placeholders, so a custom
`/set_format` can include a pixiv description. The built-in captions keep
producing byte-identical output: `compose_text` joins the two fields the
same way the single field already was, and bilibili's forward marker
(`//@author:`) now lands in `content` behind the head line's `title`.

`CachedPost.content` is `#[serde(default)]`, so link-cache entries and
queued task payloads written before the split still parse, their text
living in `title`.

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

@@ -6,6 +6,10 @@ use serde::Deserialize;
 pub struct IllustrationModel {
     pub id: u64,
     pub title: String,
+    /// The artwork's description as the app API returns it — HTML in most
+    /// works (`<br />`, `<a href>`, sometimes `<p>`), empty for many.
+    #[serde(default)]
+    pub caption: String,
     pub r#type: TypeModel,
     pub image_urls: ImageUrlsModel,
     pub user: UserInfoModel,