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>

site/api.mjs

@@ -519,6 +519,8 @@ export const ipx = {
       ['guid', 'String', ''], ['feed_id', 'String', ''], ['title', 'Option<String>', ''], ['link', 'Option<String>', ''],
       ['published', 'Option<i64>', ''],
       ['description', 'Option<String>', 'Show notes, HTML, already sanitized.'],
+      ['markdown', 'Option<String>', 'The item’s Markdown as written, when its feed sends Markdown beside its HTML (RSS’s source:markdown).'],
+      ['markdown_html', 'Option<String>', 'That Markdown made HTML, sanitized as description is.'],
       ['read', 'bool', ''], ['flagged', 'bool', 'Kept: its file is never cleared to save space.'],
       ['image', 'Option<String>', ''], ['duration', 'Option<i64>', 'Seconds.'],
       ['episode', 'Option<i64>', ''], ['season', 'Option<i64>', ''],