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>

web/src/types.ts

@@ -35,6 +35,8 @@ interface Feed {
   unread: number;
   subscribers: number;
   pinned: boolean;
+  /** From an OPML, and yours to keep should it drop the feed (#280). */
+  kept: boolean;
   private: boolean;
   private_by_address: boolean;
 }