feat: sync read/unread state to backend with persistent device ID

- APIClient: generate and persist a stable device UUID (jarvis.device.id
  in UserDefaults); attach as X-Jarvis-Device-Id header on every request
- Add markStoryRead / markStoryUnread — POST/DELETE /stories/:id/read with
  device_id, read_at, and source fields
- Wire both calls into all three read-toggle sites: SignalFeedView,
  ArticleReaderView, StoryDetailView
- Add postVoid helper for fire-and-forget POST calls (no response body needed)
- Pass include_read param on stories fetch for future server-side filtering
- CacheMaintenance: recentHours 84→72 (mirrors backend 72h story window);
  markerHours 96→192 so read suppression survives a full weekly news cycle

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

Jarvis/Store/CacheMaintenance.swift

@@ -10,8 +10,8 @@ enum CacheMaintenance {
     private static let hour: TimeInterval = 3_600
     private static let day: TimeInterval = 86_400
 
-    private static let recentHours = 84.0    // 3.5 days: survives weekends
-    private static let markerHours = 96.0   // 4 days: active window + buffer
+    private static let recentHours = 72.0    // 3 days: mirrors backend active story window
+    private static let markerHours = 192.0  // 8 days: read suppression survives weekly repeats
     private static let articleDays = 14.0    // cached article bodies (backend article retention)
 
     /// Age-based eviction. Run on launch. Cheap; protects saved stories.