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>

docs/configuration.md

@@ -124,6 +124,7 @@ folder             = "Accidental Tech Podcast"   # default: the feed title
 schedule           = "every 6h"                  # overrides [general] for this feed
 media_types        = ["audio"]                   # overrides [general] for this feed
 category           = "Technology"                # the Directory's, if the feed names none
+listed             = true                        # in the Directory with no subscribers
 username           = "ray"                       # HTTP basic auth
 password_env       = "IPX_ATP_PASS"              # preferred over a literal `password`
 ```
@@ -133,6 +134,12 @@ With more than one account, **`keywords`, `auto_download`, `allow_explicit` and
 config.toml are the fallback for a feed nobody has claimed. The keys above describe the feed itself
 and are the same for everyone. See [users.md](users.md).
 
+**The Directory** lists every feed in the catalogue, whether or not anyone subscribes to it yet.
+To put one there for others to find, `ipx add --list --category News <url>`: it subscribes no one,
+and a listed feed stays when its last subscriber leaves, where another is dropped. A feed nobody
+subscribes to is checked once a day. One that is dead (failing for 30 days) or quiet (nothing
+new in a year), with nobody subscribed and no file on disk, is removed from the catalogue.
+
 Feeds derived from a subscribed OPML are **not** in the catalogue: the OPML is the source of truth
 and they are re-derived on every scan. Editing one in the UI promotes it to a catalogue entry.