perf: move SwiftData cache and JSON encode off main thread during sync

- cacheStories() runs in Task.detached with a background ModelContext
- persistQuickCache() encodes 100 stories off-thread via Task.detached
- loadSectionSupplements() batches 5 parallel fetches into one sectionSupplement write

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

Jarvis/Views/Home/SignalFeedView.swift

@@ -10,6 +10,7 @@ struct SignalFeedView: View {
     @EnvironmentObject var ws: WebSocketManager
     @EnvironmentObject var settings: ServerSettings
     @Environment(\.modelContext) private var modelContext
+    @Environment(\.modelContainer) private var modelContainer
 
     @Query private var cachedStories: [CachedStory]
     @Query private var cachedArticles: [CachedArticle]
@@ -723,22 +724,25 @@ struct SignalFeedView: View {
 
     private func cacheStories(_ stories: [StorySummary]) {
         guard !stories.isEmpty else { return }
-        // One fetch + an in-memory index, instead of a query per story.
-        let existing = (try? modelContext.fetch(FetchDescriptor<CachedStory>())) ?? []
-        var byId = Dictionary(existing.map { ($0.id, $0) }, uniquingKeysWith: { a, _ in a })
-        for summary in stories {
-            if let found = byId[summary.id] {
-                found.signalScore = summary.signalScore
-                found.sourceCount = summary.sourceCount
-                found.tags = summary.tags ?? []
-                found.updatedAt = summary.updatedAt
-            } else {
-                let c = CachedStory(from: summary)
-                modelContext.insert(c)
-                byId[summary.id] = c
+        let container = modelContainer
+        Task.detached {
+            let ctx = ModelContext(container)
+            let existing = (try? ctx.fetch(FetchDescriptor<CachedStory>())) ?? []
+            var byId = Dictionary(existing.map { ($0.id, $0) }, uniquingKeysWith: { a, _ in a })
+            for summary in stories {
+                if let found = byId[summary.id] {
+                    found.signalScore = summary.signalScore
+                    found.sourceCount = summary.sourceCount
+                    found.tags = summary.tags ?? []
+                    found.updatedAt = summary.updatedAt
+                } else {
+                    let c = CachedStory(from: summary)
+                    ctx.insert(c)
+                    byId[summary.id] = c
+                }
             }
+            try? ctx.save()
         }
-        try? modelContext.save()
     }
 }