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/feed/Cargo.toml

@@ -19,9 +19,7 @@ quick-xml.workspace = true
 reqwest.workspace = true
 rss.workspace = true
 serde_json.workspace = true
+tokio.workspace = true
 tower.workspace = true
 tracing.workspace = true
 url.workspace = true
-
-[dev-dependencies]
-tokio.workspace = true