Add failure backoff for sync auto triggers

test/features/sync/sync_auto_trigger_controller_test.dart

@@ -30,6 +30,29 @@ void main() {
     expect(controller.lastTriggeredAt, now);
   });
 
+  test('applies backoff after failed sync attempts', () async {
+    DateTime now = DateTime(2026, 2, 15, 14);
+    bool shouldFail = true;
+
+    final SyncAutoTriggerController controller = SyncAutoTriggerController(
+      syncNow: () async => _result(success: !shouldFail),
+      now: () => now,
+      minimumInterval: const Duration(seconds: 60),
+    );
+
+    final bool first = await controller.trigger();
+    now = now.add(const Duration(seconds: 61));
+    final bool second = await controller.trigger();
+    now = now.add(const Duration(seconds: 60));
+    shouldFail = false;
+    final bool third = await controller.trigger();
+
+    expect(first, isTrue);
+    expect(second, isFalse);
+    expect(third, isTrue);
+    expect(controller.consecutiveFailures, 0);
+  });
+
   test('does not run a second sync while a run is still in progress', () async {
     int callCount = 0;
     final Completer<void> completer = Completer<void>();