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/handlers/urls.rs

@@ -327,6 +327,7 @@ pub(crate) async fn url_media(
                 &cached.author,
                 &cached.author_url,
                 &cached.title,
+                &cached.content,
                 &cached.tags,
             )
         };
@@ -406,18 +407,20 @@ pub(crate) async fn url_media(
             let caption = fetched.caption_with(&format);
             // Raw render data for the link cache; the send fills in the
             // Telegram file ids and persists the entry.
-            let cache_data = fetched
-                .render_fields()
-                .map(|(author, author_url, title, tags)| CachedPost {
-                    url: fetched.source_url.clone(),
-                    caption: fetched.caption.clone(),
-                    title: title.to_string(),
-                    author: author.to_string(),
-                    author_url: author_url.to_string(),
-                    tags: tags.to_string(),
-                    sensitive: fetched.sensitive,
-                    media: vec![],
-                });
+            let cache_data =
+                fetched
+                    .render_fields()
+                    .map(|(author, author_url, title, content, tags)| CachedPost {
+                        url: fetched.source_url.clone(),
+                        caption: fetched.caption.clone(),
+                        title: title.to_string(),
+                        content: content.to_string(),
+                        author: author.to_string(),
+                        author_url: author_url.to_string(),
+                        tags: tags.to_string(),
+                        sensitive: fetched.sensitive,
+                        media: vec![],
+                    });
             let items: Vec<MediaItemPayload> = fetched
                 .media
                 .iter()
@@ -465,6 +468,7 @@ 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(),