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>

web/src/types.ts

@@ -49,6 +49,9 @@ interface Entry {
   link: string | null;
   published: number | null;
   description: string | null;
+  /** The item's Markdown as written, when its feed sends it beside the HTML, and made HTML (#285). */
+  markdown: string | null;
+  markdown_html: string | null;
   read: boolean;
   flagged: boolean;
   image: string | null;