feat(bg): add background feed refresh every ~30 min

Registers com.kisani.jarvis.feed.refresh as a BGAppRefreshTask.
On each fire it fetches the latest 40 stories and upserts CachedStory
rows so the SwiftData cache is warm before the user opens the app.
Already-cached rows have signalScore / sourceCount refreshed in-place.

Single ModelContainer owned by JarvisApp and shared via
BackgroundRefreshManager.container to avoid multi-container conflicts.
BG task ID registered in AppDelegate and declared in project.yml.

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

project.yml

@@ -34,6 +34,7 @@ targets:
           - processing
         BGTaskSchedulerPermittedIdentifiers:
           - com.kisani.jarvis.briefing.refresh
+          - com.kisani.jarvis.feed.refresh
     settings:
       base:
         PRODUCT_BUNDLE_IDENTIFIER: com.kisani.jarvis