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/ipx/CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- An item whose feed sends Markdown beside its HTML, as Scripting News does with `<source:markdown>`, keeps it: `markdown` and `markdown_html` in the API's items (#285).
 - An item written in Markdown shows formatted, not as Markdown's raw characters: a description in Markdown, or, with no other text, RSS's `<source:markdown>`. HTML a feed sends is left as it is (#284).
 - A show from an OPML subscription or a Patreon creator can be kept: `kept` on `PATCH /api/feeds/{id}` and in the feed list. When the OPML drops a kept show, or you leave the OPML, it stays on your list as a feed of its own (#280).
 - `/metrics` counts Signals: items judged and fitting, calls to Jev by how they ended, the time they took and the tokens TypeSafe counted, time since the judge's last pass, and keys refused; a failed call to Jev shows as a failed trace (#270, #275).