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/xmedia-bot/src/link_cache.rs

@@ -38,6 +38,10 @@ pub struct CachedPost {
     /// override).
     pub caption: String,
     pub title: String,
+    /// The post's body text. Defaulted on read: entries written before the
+    /// title/content split carry it inside `title`.
+    #[serde(default)]
+    pub content: String,
     pub author: String,
     pub author_url: String,
     pub tags: String,
@@ -182,6 +186,7 @@ mod tests {
             url: "https://x.com/u/status/1".into(),
             caption: "cap".into(),
             title: "t".into(),
+            content: "c".into(),
             author: "a".into(),
             author_url: "au".into(),
             tags: "".into(),