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/config.rs

@@ -163,6 +163,10 @@ pub struct Feed {
     /// Preferred over `password`: name of an env var holding the password.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub password_env: Option<String>,
+    /// Put in the Directory by an admin (`ipx add --list`): listed with no subscribers, and kept
+    /// in the catalogue when the last one leaves, where anyone else's feed is dropped.
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub listed: bool,
 }
 
 fn yes() -> bool {
@@ -584,7 +588,7 @@ mod tests {
         taken.insert("the-daily".to_string(), Feed {
             url: "u".into(), folder: None, 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,
+            password: None, password_env: None, category: None, listed: false,
         });
         assert_eq!(unique_slug("The Daily", &taken), "the-daily-2");
     }
@@ -605,6 +609,7 @@ mod tests {
             password: Some("literal".into()),
             password_env: None,
             category: None,
+            listed: false,
         };
         assert_eq!(f.password().as_deref(), Some("literal"));