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/Store/StoryStore.swift

@@ -45,12 +45,16 @@ final class StoryStore: ObservableObject {
     }
 
     /// Persist the current feed to UserDefaults so it survives process kills.
+    /// Encoding runs off the main thread; UserDefaults.set is thread-safe.
     private func persistQuickCache() {
-        let encoder = JSONEncoder()
-        encoder.dateEncodingStrategy = .iso8601
         let payload = QuickCachePayload(stories: Array(stories.prefix(100)), savedAt: Date())
-        if let data = try? encoder.encode(payload) {
-            UserDefaults.standard.set(data, forKey: quickCacheKey)
+        let key = quickCacheKey
+        Task.detached {
+            let encoder = JSONEncoder()
+            encoder.dateEncodingStrategy = .iso8601
+            if let data = try? encoder.encode(payload) {
+                UserDefaults.standard.set(data, forKey: key)
+            }
         }
     }
 
@@ -219,15 +223,24 @@ final class StoryStore: ObservableObject {
 
     func loadSectionSupplements() {
         guard selectedTags.isEmpty else { return }   // only for "All" pill
-        for sec in supplementSections {
-            let id = sec.id
-            let tags = sec.tags
-            Task { [weak self] in
-                guard let self else { return }
-                if let result = try? await api.fetchStories(tags: tags, limit: 8) {
-                    await MainActor.run { self.sectionSupplement[id] = result.data }
+        let sections = supplementSections
+        Task { [weak self] in
+            guard let self else { return }
+            var merged: [String: [StorySummary]] = [:]
+            await withTaskGroup(of: (String, [StorySummary]?).self) { group in
+                for sec in sections {
+                    let id = sec.id
+                    let tags = sec.tags
+                    group.addTask {
+                        let result = try? await APIClient.shared.fetchStories(tags: tags, limit: 8)
+                        return (id, result?.data)
+                    }
+                }
+                for await (id, data) in group {
+                    if let data { merged[id] = data }
                 }
             }
+            self.sectionSupplement = merged   // single main-thread update
         }
     }