Items written in Markdown show formatted (#284)

A description with Markdown and no HTML tag is rendered to HTML with
pulldown-cmark, and an item with no text but RSS's <source:markdown>
(https://source.scripting.com/, as Scripting News sends beside its HTML) uses
that. HTML is left alone, and plain text without Markdown in it stays as it
was, so a line like "1984. ..." is not made a list. ammonia cleans the result
as any feed's HTML. Stored items keep what they have: the scanner does not
rewrite an item it has.

Reference: http://scripting.com/2026/10/06/005559.html

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 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. For items new from now on (#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).
 - An admin setting for how many days back a new Signal reads (#276).