TODO.md: show a publisher's error in the UI

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Dcx59boh4pasuNwAVU7un

TODO.md

@@ -27,6 +27,16 @@ User-Agent; a browser gets the same answers.
   the new feed for om.co, ms.now, Letters of Note, the Daily Dot, Hell Gate, The Frame Lab and
   Daily Kos. A `202` with an empty body (British Antarctic Survey) should read as "nothing yet", not
   as a parse failure. (`src/feed.rs`)
+- [ ] **Show a publisher's error in the UI.** Today a failing feed shows its raw error in red only
+  once you open it (`web/index.html:877`, `:960`); the OPML view marks a failing child "error"
+  (`:988`), and the sidebar shows nothing. Mark a failing feed in the sidebar too, and say whose
+  problem it is and what to do, in plain words: a 404 means the publisher took the feed down or moved
+  it, 401 and 403 that the site refuses ipx, 402 that the feed needs a paid plan, a name that no
+  longer resolves that the site is gone, and a web page instead of a feed that it moved (with the new
+  URL when the page links one). Offer Unsubscribe, or Use the new address. Keep timeouts, 5xx and
+  429 out of it, and flag a feed only once it has failed for a day: macmanx failed once and read
+  fine an hour later. That needs the time a feed started failing, kept beside `last_error` and
+  cleared when a check succeeds. (`web/index.html`, `src/db.rs`, `src/main.rs`)
 - [ ] **Westenberg.** The only failing feed anyone subscribes to. The Hacker News stars list still
   gives `joanwestenberg.com/rss`, which is a 404; the feed is now `joanwestenberg.com/feed`.
   Nothing for ipx to fix; subscribe to the new URL directly.