Add cache cleanup: age-based auto-eviction + manual Clear cache

CacheMaintenance prunes the SwiftData caches so they don't grow forever:
seen markers >30d, read >90d, cached articles >30d, cached stories >14d —
always protecting saved bookmarks. Runs on launch. Settings gains a
"Clear offline cache" action (keeps saved) with a count + confirmation.

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

Jarvis/Views/Home/SignalFeedView.swift

@@ -120,6 +120,7 @@ struct SignalFeedView: View {
         .onChange(of: store.stories) { _, newValue in
             cacheStories(newValue)
         }
+        .task { CacheMaintenance.prune(modelContext) }   // bound the caches on launch
         .task(id: selectedPill) {
             // A filter active but nothing matched in the loaded set → pull a few
             // more pages so sparse pills still populate.