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>

ExportOptions.plist

@@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <!--
+        method: "app-store" to upload to TestFlight/App Store.
+                "development" for ad-hoc device installs.
+                "ad-hoc" for distribution outside the App Store.
+    -->
+    <key>method</key>
+    <string>app-store</string>
+
+    <key>teamID</key>
+    <string>K8BLMMR883</string>
+
+    <!--
+        destination: "upload" sends directly to App Store Connect after export.
+                     "export" writes the IPA locally only (upload manually or via altool).
+    -->
+    <key>destination</key>
+    <string>export</string>
+
+    <key>signingStyle</key>
+    <string>automatic</string>
+
+    <key>stripSwiftSymbols</key>
+    <true/>
+
+    <key>uploadBitcode</key>
+    <false/>
+
+    <key>compileBitcode</key>
+    <false/>
+
+    <key>thinning</key>
+    <string>&lt;none&gt;</string>
+</dict>
+</plist>