Cache backend tags so offline pill filtering stays accurate

Add tags[] to CachedStory (defaulted for lightweight migration) and persist
them on upsert; offline StorySummary now carries the cached tags instead of
falling back to topic. Backend stays the source of truth.

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

Jarvis/Views/Home/SignalFeedView.swift

@@ -466,6 +466,7 @@ struct SignalFeedView: View {
             if let found = existing?.first {
                 found.signalScore = summary.signalScore
                 found.sourceCount = summary.sourceCount
+                found.tags = summary.tags ?? []
                 found.updatedAt = summary.updatedAt
             } else {
                 modelContext.insert(CachedStory(from: summary))
@@ -483,7 +484,7 @@ extension StorySummary {
             headline: cached.headline,
             summary: cached.summary,
             topic: cached.topic,
-            tags: nil,
+            tags: cached.tags,
             signalScore: cached.signalScore,
             scoreBreakdown: ScoreBreakdown(sourceAuthority: 0, freshness: 0, localRelevance: 0,
                                            crossSourceConfirmation: 0, topicImportance: 0),