A YouTube feed's spurious 404 or 500 is asked again (#382)

YouTube answers some requests for a channel's feed with a 404 or 500
in tens of milliseconds, while the next request for the same feed, or
another channel's 200 ms later, reads. Checked once a day, the 39
YouTube channels in the catalogue missed most days: 178 failures in
the week to 2026-11-04, against 74 in the ten days before, and all 39
read from Tower when asked by hand.

feed::fetch now asks a youtube.com/feeds/videos.xml address again, up
to twice, a second apart, when it answers 404 or 5xx. Other hosts'
404s are real and are not retried. tokio moves from ipx-feed's dev
dependencies to its own, for the sleep; both services build it already.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/directory/CHANGELOG.md

@@ -20,6 +20,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Its JSON log lines no longer carry their span, once on its own and again in a list, which said nothing the line's own fields did not (#363).
 
+### Fixed
+
+- A YouTube channel's check no longer fails on the 404 or 500 YouTube answers some feed requests with and does not mean: it is asked again, twice, a second apart (#382).
+
 ## [1.4.1] - 2026-10-24
 
 ### Fixed