List feeds in the Directory before anyone subscribes, and clear out dead ones (#107)

The Directory showed a catalogue feed only once someone subscribed, and a feed left the
catalogue with its last subscriber, so nothing could be put there for others to find.

- A feed has a listed flag, set by ipx add --list (with --category for the Directory's chip).
  The web page keeps a listed feed in the catalogue when its last subscriber leaves.
- The Directory lists every catalogue feed; Popular still only what people subscribe to.
  popular() reads titles, artwork and categories through Db::feed_list, not three queries a
  feed. Subscribing from the Directory scans the feed at once.
- A feed nobody subscribes to is checked once a day at most.
- clean_directory, in the sweep before each scan, removes from the catalogue and the database
  a feed nobody subscribes to, with no file on disk and not from an OPML, that has failed for
  30 days or published nothing in a year. Run against production first: it removes nothing.

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

src/download.rs

@@ -406,7 +406,7 @@ mod tests {
         let mut f = crate::config::Feed {
             url: "u".into(), folder: Some("Subscriptions/Some | Show".into()), group: None, media_types: None,
             schedule: None, keywords: vec![], allow_explicit: false, auto_download: true,
-            max_new_per_check: None, username: None, password: None, password_env: None, category: None,
+            max_new_per_check: None, username: None, password: None, password_env: None, category: None, listed: false,
         };
         assert_eq!(folder_for(&cfg, "id", &f, None), "Subscriptions/Some - Show");