Sink seen/read stories, fix F1 matching, drop East Africa pill

- Sink already-seen and read stories to the bottom of the feed (new SeenStory
  model + per-load snapshot so order is stable while scrolling and refreshes
  sink what you've seen) so fresh content surfaces.
- F1 matching: clear F1 headline OR a majority of F1-feed sources, replacing the
  broad keyword list that leaked crypto in (fia -> "deFIAnt"). Dropped unused
  matchSources path.
- Remove the East Africa pill.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Jarvis/Models/Models.swift

@@ -210,6 +210,17 @@ final class SavedStory {
     }
 }
 
+@Model
+final class SeenStory {
+    @Attribute(.unique) var id: String
+    var seenAt: Date
+
+    init(id: String) {
+        self.id = id
+        self.seenAt = Date()
+    }
+}
+
 @Model
 final class CachedArticle {
     @Attribute(.unique) var id: String