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/bsky/interface.rs

@@ -330,13 +330,16 @@ impl From<Post> for Fetched {
             url: url.clone(),
             author: encode_text(&post.author).into_owned(),
             author_url: author_url.clone(),
-            title: encode_text(&post.text).into_owned(),
+            // A post has no title: its text is all content.
+            title: String::new(),
+            content: encode_text(&post.text).into_owned(),
             tags: String::new(),
         });
         Fetched {
             source_url: url,
             caption: post.caption(),
-            title: post.text.clone(),
+            title: String::new(),
+            content: post.text.clone(),
             media: post.media,
             sensitive: post.sensitive,
             site_id: "bsky",
@@ -410,7 +413,8 @@ mod tests {
             fetched.source_url,
             "https://bsky.app/profile/user.bsky.social/post/3xxxx"
         );
-        assert_eq!(fetched.title, "hello <world>");
+        assert_eq!(fetched.title, "");
+        assert_eq!(fetched.content, "hello <world>");
         assert_eq!(fetched.media.len(), 1);
         assert!(!fetched.sensitive);
         // display_name absent -> empty fallback