feat: offline article caching + smart pull-to-refresh reshuffle

Offline caching:
- BackgroundRefreshManager.preCacheArticles() fetches top article per story
  (score ≥60, up to 20 stories) and stores as CachedArticle in SwiftData
- Runs after every BG fetch AND after every foreground sync via StoryStore
- Articles are available in ArticleReaderView offline without ever opening them

Pull-to-refresh reshuffle when nothing new:
- loadStories(refresh:) now returns newCount (genuinely new story IDs)
- If server returns 0 new stories, feed merges in any BG-cached stories not
  already in the live feed, then re-sorts by signal score (reshuffle)
- seenSnapshot is always cleared so everything re-ranks fresh

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Jarvis/Store/BackgroundRefreshManager.swift

@@ -65,6 +65,9 @@ enum BackgroundRefreshManager {
 
             try? context.save()
 
+            // Pre-cache articles for offline reading
+            await preCacheArticles(page.data, container: container)
+
             if !newStories.isEmpty {
                 notifyNewStories(newStories)
             }
@@ -73,6 +76,30 @@ enum BackgroundRefreshManager {
         }
     }
 
+    // MARK: - Article pre-caching
+
+    /// Fetch and store the top article for each high-signal uncached story so
+    /// the user can read them offline without having opened them first.
+    /// Safe to call in background or foreground — creates its own ModelContext.
+    static func preCacheArticles(_ stories: [StorySummary], container: ModelContainer) async {
+        let context = ModelContext(container)
+        let top = stories
+            .filter { $0.signalScore >= 60 && !$0.sources.isEmpty }
+            .sorted { $0.signalScore > $1.signalScore }
+            .prefix(20)
+
+        for story in top {
+            let sid = story.id
+            let alreadyCached = (try? context.fetch(
+                FetchDescriptor<CachedArticle>(predicate: #Predicate { $0.storyId == sid })
+            ))?.isEmpty == false
+            guard !alreadyCached, let src = story.sources.first else { continue }
+            guard let art = try? await APIClient.shared.fetchArticle(id: src.id) else { continue }
+            context.insert(CachedArticle(from: art))
+            try? context.save()
+        }
+    }
+
     // MARK: - New-story notifications
 
     // Slugs for the featured categories the user cares about.