Add push-equivalent notifications via background polling; bump to 3.6

Sideloaded apps can't use APNs (no push entitlement without Apple signing),
so deliver notifications via an iOS BGAppRefreshTask that polls the monitored
repos every ~15 min and fires local notifications (no Apple push needed):
- new PRs that appear in the open set
- CI status changes on existing PRs

Persistence: NotificationState tracks announced PR ids + last-known CI status
so we only fire on actual changes, not every poll. AppDelegate registers the
background task and reschedules the next one. Info.plist gets UIBackgroundModes
fetch + the BGTask identifier. Settings gains a Notifications toggle; also
poll after a foreground inbox load so state seeds and changes are caught
promptly. Bump to 3.6.

GiteaPR/AppDelegate.swift

@@ -0,0 +1,50 @@
+import UIKit
+import BackgroundTasks
+
+/// App delegate that schedules the background refresh task which drives the
+/// push-equivalent notifications (polling the monitored repos).
+///
+/// iOS runs BGAppRefreshTask opportunistically (~every 15 min at best, often
+/// slower), and only when the app has been used recently and isn't force-quit.
+final class AppDelegate: NSObject, UIApplicationDelegate {
+    static let taskIdentifier = "com.bueso.giteapr.refresh"
+
+    func application(_ application: UIApplication,
+                     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]? = nil) -> Bool {
+        registerBackgroundTask()
+        return true
+    }
+
+    private func registerBackgroundTask() {
+        BGTaskScheduler.shared.register(
+            forTaskWithIdentifier: Self.taskIdentifier,
+            using: nil
+        ) { task in
+            // Handle the background refresh.
+            self.handle(task)
+        }
+    }
+
+    private func handle(_ task: BGTask) {
+        // Reschedule the next refresh so the chain continues.
+        scheduleNextRefresh()
+
+        let pollTask = Task { @MainActor in
+            await NotificationService.shared.poll()
+        }
+        task.expirationHandler = {
+            pollTask.cancel()
+        }
+        Task {
+            await pollTask.value
+            task.setTaskCompleted(success: true)
+        }
+    }
+
+    /// Schedule the next background refresh (~15 min from now, earliest).
+    func scheduleNextRefresh() {
+        let request = BGAppRefreshTaskRequest(identifier: Self.taskIdentifier)
+        request.earliestBeginDate = Date(timeIntervalSinceNow: 15 * 60)
+        try? BGTaskScheduler.shared.submit(request)
+    }
+}