Switch an item between its HTML and its Markdown (#285)

A feed that sends <source:markdown> beside its HTML (Scripting News) has its
Markdown kept in entries.markdown, added to existing databases by an ALTER.
A stored item gets it when a scan finds it told differently, the Markdown
counting as a change, the way corrections arrive (#141). The API sends it as
written and made HTML, cleaned by ammonia like the description.

The item's page has a choice between its meta line and its buttons: HTML,
Markdown or Markdown source, remembered in the browser.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/feed/src/lib.rs

@@ -27,6 +27,9 @@ pub struct Entry {
     pub link: Option<String>,
     pub published: Option<i64>,
     pub description: Option<String>,
+    /// The item's Markdown, when the feed sends it beside HTML of its own (`<source:markdown>`):
+    /// the page can show either (#285). With no HTML it is the description instead (#284).
+    pub markdown: Option<String>,
     pub categories: Vec<String>,
     pub explicit: bool,
     /// Episode artwork; falls back to the feed's in the UI.
@@ -770,6 +773,8 @@ fn from_rss(ch: rss::Channel, bytes: &[u8]) -> ParsedFeed {
                 // Content wins over description, as __getEntries preferred entry.content. The
                 // source namespace's Markdown copy only when there is neither (#284).
                 description: notes(item.content(), item.description(), source_markdown(item)),
+                markdown: non_empty(source_markdown(item))
+                    .filter(|_| non_empty(item.content()).or_else(|| non_empty(item.description())).is_some()),
                 categories: item
                     .categories()
                     .iter()
@@ -839,6 +844,7 @@ fn from_atom(feed: atom_syndication::Feed) -> ParsedFeed {
                 link: alt.map(str::to_owned),
                 published: e.published().or(Some(e.updated())).map(|d| d.timestamp()),
                 description: notes(e.content().and_then(|c| c.value()), e.summary().map(|s| s.as_str()), None),
+                markdown: None,
                 categories: e.categories().iter().map(|c| c.term().to_owned()).collect(),
                 explicit: false,
                 image: None,
@@ -966,7 +972,8 @@ fn is_markdown(s: &str) -> bool {
     !tag && (s.contains("](") || s.contains("**") || s.lines().any(block))
 }
 
-fn markdown_html(md: &str) -> String {
+/// Markdown made HTML, uncleaned: whoever shows it cleans it as any feed's HTML.
+pub fn markdown_html(md: &str) -> String {
     use pulldown_cmark::{Options, Parser, html};
     let mut out = String::new();
     html::push_html(&mut out, Parser::new_ext(md, Options::ENABLE_TABLES | Options::ENABLE_STRIKETHROUGH));
@@ -1423,6 +1430,15 @@ Some **bold** and [a link](https://x.example/).</description></item>
         assert_eq!(got[3].as_deref(), Some("1984. Plain text, left as it is."));
     }
 
+    #[test]
+    fn markdown_sent_beside_html_is_kept_for_the_page_to_switch_to() {
+        let xml = br#"<?xml version="1.0"?><rss version="2.0" xmlns:source="https://source.scripting.com/"><channel><title>R</title>
+        <item><guid>both</guid><description>&lt;p&gt;Hi&lt;/p&gt;</description><source:markdown>*Hi*</source:markdown></item>
+        <item><guid>alone</guid><source:markdown>*Hi*</source:markdown></item></channel></rss>"#;
+        let got: Vec<_> = parse(xml).unwrap().entries.into_iter().map(|e| e.markdown).collect();
+        assert_eq!(got, [Some("*Hi*".to_owned()), None], "only beside HTML; alone, it is the description (#284)");
+    }
+
     #[test]
     fn a_file_wordpress_lists_twice_is_one_enclosure() {
         let xml = br#"<?xml version="1.0"?><rss version="2.0"><channel><title>R</title>