Keep a show from an OPML when the OPML drops it (#280)

A show inside a subscribed OPML went when the OPML stopped listing it
(orphaned only if it had downloads), and went for you when you left the
OPML. Keep, in its settings, is a per-person flag on the subscription:
when the OPML drops a kept show, or nobody subscribes to the OPML any more,
it becomes a catalogue entry outside the OPML, taking the OPML's settings
while it is there, and stays on the lists of those who kept it. Whoever had
it only through the OPML loses it as before. Leaving the OPML yourself
leaves what you kept on your list.

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

crates/ipx/CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- A show from an OPML subscription or a Patreon creator can be kept: `kept` on `PATCH /api/feeds/{id}` and in the feed list. When the OPML drops a kept show, or you leave the OPML, it stays on your list as a feed of its own (#280).
 - `/metrics` counts Signals: items judged and fitting, calls to Jev by how they ended, the time they took and the tokens TypeSafe counted, time since the judge's last pass, and keys refused; a failed call to Jev shows as a failed trace (#270, #275).
 - An admin setting for how many days back a new Signal reads (#276).
 - `/metrics` breaks Signals' items, fits and TypeSafe tokens out by person and by whose key paid, so the server's bill can be split (#278).