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>

Cargo.lock

@@ -1931,6 +1931,7 @@ dependencies = [
  "atom_syndication",
  "chrono",
  "opml",
+ "pulldown-cmark",
  "quick-xml 0.42.0",
  "reqwest",
  "rss",
@@ -3128,6 +3129,24 @@ dependencies = [
  "syn 2.0.119",
 ]
 
+[[package]]
+name = "pulldown-cmark"
+version = "0.13.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e9f068eba8e7071c5f9511831b44f32c740d5adf574e990f946ddb53db2f314e"
+dependencies = [
+ "bitflags 2.13.1",
+ "memchr",
+ "pulldown-cmark-escape",
+ "unicase",
+]
+
+[[package]]
+name = "pulldown-cmark-escape"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "007d8adb5ddab6f8e3f491ac63566a7d5002cc7ed73901f72057943fa71ae1ae"
+
 [[package]]
 name = "quanta"
 version = "0.12.6"