refactor(shopinbit): consolidate poll backoff into the client

diff --git a/lib/pages/shopinbit/shopinbit_car_research_payment_view.dart b/lib/pages/shopinbit/shopinbit_car_research_payment_view.dart
index 08c81eb..2896158 100644
--- a/lib/pages/shopinbit/shopinbit_car_research_payment_view.dart
+++ b/lib/pages/shopinbit/shopinbit_car_research_payment_view.dart
@@ -8,6 +8,7 @@ import '../../app_config.dart';
 import '../../notifications/show_flush_bar.dart';
 import '../../providers/global/shopin_bit_service_provider.dart';
 import '../../providers/providers.dart';
+import '../../services/shopinbit/src/client.dart';
 import '../../services/shopinbit/src/models/car_research.dart';
 import '../../themes/stack_colors.dart';
 import '../../utilities/assets.dart';
@@ -229,11 +230,6 @@ class _ShopInBitCarResearchPaymentViewState
     _pollTimer = Timer(_pollInterval, _pollTick);
   }
 
-  Duration _nextBackoff(Duration current) {
-    final Duration next = current * 2;
-    return next > _kMaxPollInterval ? _kMaxPollInterval : next;
-  }
-
   /// Periodic driver: poll once, then reschedule with backoff on failure and
   /// reset on success. Stops once the flow is terminal or finalizing.
   Future<void> _pollTick() async {
@@ -244,7 +240,9 @@ class _ShopInBitCarResearchPaymentViewState
         _flowState == _PaymentFlowState.complete) {
       return;
     }
-    _pollInterval = ok ? _kBasePollInterval : _nextBackoff(_pollInterval);
+    _pollInterval = ok
+        ? _kBasePollInterval
+        : ShopInBitClient.nextPollBackoff(_pollInterval, _kMaxPollInterval);
     _scheduleNextPoll();
   }
 
diff --git a/lib/pages/shopinbit/shopinbit_payment_view.dart b/lib/pages/shopinbit/shopinbit_payment_view.dart
index f306982..2934a97 100644
--- a/lib/pages/shopinbit/shopinbit_payment_view.dart
+++ b/lib/pages/shopinbit/shopinbit_payment_view.dart
@@ -10,6 +10,7 @@ import '../../app_config.dart';
 import '../../notifications/show_flush_bar.dart';
 import '../../providers/global/shopin_bit_service_provider.dart';
 import '../../providers/providers.dart';
+import '../../services/shopinbit/src/client.dart';
 import '../../services/shopinbit/src/models/payment.dart';
 import '../../themes/coin_icon_provider.dart';
 import '../../themes/stack_colors.dart';
@@ -132,11 +133,6 @@ class _ShopInBitPaymentViewState extends ConsumerState<ShopInBitPaymentView>
     _pollTimer = Timer(_pollInterval, _pollPayment);
   }
 
-  Duration _nextBackoff(Duration current) {
-    final Duration next = current * 2;
-    return next > _kMaxPollInterval ? _kMaxPollInterval : next;
-  }
-
   Future<void> _pollPayment() async {
     bool ok = false;
     try {
@@ -164,7 +160,9 @@ class _ShopInBitPaymentViewState extends ConsumerState<ShopInBitPaymentView>
     }
     // Back off on failure (e.g. a 429), reset to base on success, so a rate
     // limit slows us down instead of getting hammered every 15s.
-    _pollInterval = ok ? _kBasePollInterval : _nextBackoff(_pollInterval);
+    _pollInterval = ok
+        ? _kBasePollInterval
+        : ShopInBitClient.nextPollBackoff(_pollInterval, _kMaxPollInterval);
     _scheduleNextPoll();
   }
 
diff --git a/lib/pages/shopinbit/shopinbit_ticket_detail.dart b/lib/pages/shopinbit/shopinbit_ticket_detail.dart
index 49fac0e..2918d54 100644
--- a/lib/pages/shopinbit/shopinbit_ticket_detail.dart
+++ b/lib/pages/shopinbit/shopinbit_ticket_detail.dart
@@ -9,6 +9,7 @@ import 'package:intl/intl.dart';
 import '../../db/drift/shared_db/shared_database.dart';
 import '../../models/shopinbit/shopinbit_enums.dart';
 import '../../providers/global/shopin_bit_service_provider.dart';
+import '../../services/shopinbit/src/client.dart';
 import '../../services/shopinbit/src/models/message.dart';
 import '../../services/shopinbit/src/models/ticket.dart';
 import '../../themes/stack_colors.dart';
@@ -111,15 +112,12 @@ class _ShopInBitTicketDetailState extends ConsumerState<ShopInBitTicketDetail>
     }
 
     // Back off on failure (e.g. a 429), reset on success.
-    _pollInterval = ok ? _kBasePollInterval : _nextBackoff(_pollInterval);
+    _pollInterval = ok
+        ? _kBasePollInterval
+        : ShopInBitClient.nextPollBackoff(_pollInterval, _kMaxPollInterval);
     _pollingTimer = Timer(_pollInterval, _poll);
   }
 
-  Duration _nextBackoff(Duration current) {
-    final Duration next = current * 2;
-    return next > _kMaxPollInterval ? _kMaxPollInterval : next;
-  }
-
   void _startPolling() {
     _pollingTimer?.cancel();
     _pollInterval = _kBasePollInterval;
diff --git a/lib/services/shopinbit/src/client.dart b/lib/services/shopinbit/src/client.dart
index 8eb3855..42981d1 100644
--- a/lib/services/shopinbit/src/client.dart
+++ b/lib/services/shopinbit/src/client.dart
@@ -632,6 +632,12 @@ class ShopInBitClient {
     }
   }
 
+  /// Next poll interval after a failed poll: double [current], capped at [max].
+  static Duration nextPollBackoff(Duration current, Duration max) {
+    final Duration next = current * 2;
+    return next > max ? max : next;
+  }
+
   /// How long to wait before retrying a 429. Prefers a sane `Retry-After`
   /// header; otherwise 1s, 2s, 4s... with jitter, capped at [_kMaxBackoff].
   Duration _backoffDelay(int attempt, Map<String, String> headers) {