Add failure backoff for sync auto triggers

lib/features/sync/sync_auto_trigger_controller.dart

@@ -1,3 +1,5 @@
+import 'dart:math' as math;
+
 import 'package:flutter/foundation.dart';
 import 'package:relationship_saver/features/sync/sync_coordinator.dart';
 
@@ -18,6 +20,7 @@ class SyncAutoTriggerController {
 
   bool _running = false;
   DateTime? _lastTriggeredAt;
+  int _consecutiveFailures = 0;
 
   @visibleForTesting
   bool get isRunning => _running;
@@ -25,6 +28,9 @@ class SyncAutoTriggerController {
   @visibleForTesting
   DateTime? get lastTriggeredAt => _lastTriggeredAt;
 
+  @visibleForTesting
+  int get consecutiveFailures => _consecutiveFailures;
+
   /// Triggers `syncNow` when not already running and outside cooldown window.
   Future<bool> trigger({bool ignoreCooldown = false}) async {
     if (_running) {
@@ -35,19 +41,34 @@ class SyncAutoTriggerController {
     final DateTime? last = _lastTriggeredAt;
     if (!ignoreCooldown &&
         last != null &&
-        now.difference(last) < minimumInterval) {
+        now.difference(last) < _cooldownForCurrentState()) {
       return false;
     }
 
     _running = true;
     _lastTriggeredAt = now;
     try {
-      await _syncNow();
+      final SyncRunResult result = await _syncNow();
+      if (result.success) {
+        _consecutiveFailures = 0;
+      } else {
+        _consecutiveFailures += 1;
+      }
       return true;
     } catch (_) {
+      _consecutiveFailures += 1;
       return true;
     } finally {
       _running = false;
     }
   }
+
+  Duration _cooldownForCurrentState() {
+    if (_consecutiveFailures <= 0) {
+      return minimumInterval;
+    }
+    final int cappedFailures = math.min(_consecutiveFailures, 4);
+    final int multiplier = 1 << cappedFailures;
+    return minimumInterval * multiplier;
+  }
 }