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/twitter/auth.rs

@@ -327,7 +327,8 @@ mod tests {
             "https://x.com/nsfw_author/status/2083868672721039569"
         );
         // The appended media short link (no URL-entity mapping) is stripped.
-        assert_eq!(fetched.title, "nsfw content");
+        assert_eq!(fetched.title, "");
+        assert_eq!(fetched.content, "nsfw content");
     }
 
     #[test]

crates/x-media/src/site/twitter/interface.rs

@@ -99,6 +99,7 @@ fn empty_fetched(url: &str) -> Fetched {
         // crafted links cannot break the parse (Telegram 400).
         caption: encode_text(url).into_owned(),
         title: String::new(),
+        content: String::new(),
         media: vec![],
         sensitive: true,
         site_id: "twitter",
@@ -352,17 +353,20 @@ impl From<Tweet> for Fetched {
     fn from(tweet: Tweet) -> Self {
         let url = tweet.url();
         let author_url = tweet.author_url();
+        // A tweet has no title: its text is all content.
         let render_data = Some(crate::site::RenderData {
             url: url.clone(),
             author: encode_text(&tweet.author).into_owned(),
             author_url: author_url.clone(),
-            title: encode_text(&tweet.text).into_owned(),
+            title: String::new(),
+            content: encode_text(&tweet.text).into_owned(),
             tags: String::new(),
         });
         Fetched {
             source_url: url,
             caption: tweet.caption(),
-            title: tweet.text.clone(),
+            title: String::new(),
+            content: tweet.text.clone(),
             media: tweet.media,
             sensitive: tweet.sensitive,
             site_id: "twitter",
@@ -437,7 +441,8 @@ mod tests {
         assert_eq!(tweet.text, ">^ω^< & more 'quoted'");
         assert_eq!(tweet.author, "O'Brien");
         let fetched: Fetched = tweet.into();
-        assert_eq!(fetched.title, ">^ω^< & more 'quoted'");
+        assert_eq!(fetched.title, "");
+        assert_eq!(fetched.content, ">^ω^< & more 'quoted'");
         // The caption escapes the raw text exactly once (encode_text covers
         // & < >; apostrophes stay literal — they are harmless in text).
         assert!(
@@ -496,7 +501,8 @@ mod tests {
             fetched.source_url,
             "https://x.com/author_handle/status/861627479294746624"
         );
-        assert_eq!(fetched.title, "a & b <c>");
+        assert_eq!(fetched.title, "");
+        assert_eq!(fetched.content, "a & b <c>");
         assert!(fetched.sensitive);
         assert_eq!(fetched.media.len(), 2);
         match &fetched.media[0] {