feat: add local-first private AI digest workflow

Migrate app code into canonical feature slices, add phone-only AI digest scheduling and review, wire local notification/background task support, and cover the flow with tests.

ios/Runner/AppDelegate.swift

@@ -1,5 +1,6 @@
 import Flutter
 import UIKit
+import workmanager_apple
 
 @main
 @objc class AppDelegate: FlutterAppDelegate, FlutterImplicitEngineDelegate {
@@ -7,6 +8,10 @@ import UIKit
     _ application: UIApplication,
     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
   ) -> Bool {
+    WorkmanagerPlugin.registerPeriodicTask(
+      withIdentifier: "com.relationshipsaver.llm.digest",
+      frequency: NSNumber(value: 7 * 24 * 60 * 60)
+    )
     return super.application(application, didFinishLaunchingWithOptions: launchOptions)
   }