feat: Phase 1 critical gaps — gamification API, Razorpay checkout, Google OAuth, notifications

- Fix gamification endpoints to use Node.js server (app.eventifyplus.com)
- Replace 6 mock gamification methods with real API calls (dashboard, leaderboard, shop, redeem, submit)
- Add booking models, service, payment service (Razorpay), checkout provider
- Add 3-step CheckoutScreen with Razorpay native modal integration
- Add Google OAuth login (Flutter + Django backend)
- Add full notifications system (Django model + 3 endpoints + Flutter UI)
- Register CheckoutProvider, NotificationProvider in main.dart MultiProvider
- Wire notification bell in HomeScreen app bar
- Add razorpay_flutter ^1.3.7 and google_sign_in ^6.2.2 packages

lib/features/booking/services/payment_service.dart

@@ -0,0 +1,67 @@
+// lib/features/booking/services/payment_service.dart
+
+import 'package:razorpay_flutter/razorpay_flutter.dart';
+import 'package:flutter/foundation.dart';
+
+typedef PaymentSuccessCallback = void Function(PaymentSuccessResponse response);
+typedef PaymentErrorCallback = void Function(PaymentFailureResponse response);
+typedef ExternalWalletCallback = void Function(ExternalWalletResponse response);
+
+class PaymentService {
+  late Razorpay _razorpay;
+
+  // Razorpay test key — matches web app
+  static const String _testKey = 'rzp_test_S49PVZmqAVoWSH';
+
+  PaymentSuccessCallback? onSuccess;
+  PaymentErrorCallback? onError;
+  ExternalWalletCallback? onExternalWallet;
+
+  void initialize({
+    required PaymentSuccessCallback onSuccess,
+    required PaymentErrorCallback onError,
+    ExternalWalletCallback? onExternalWallet,
+  }) {
+    _razorpay = Razorpay();
+    this.onSuccess = onSuccess;
+    this.onError = onError;
+    this.onExternalWallet = onExternalWallet;
+
+    _razorpay.on(Razorpay.EVENT_PAYMENT_SUCCESS, _handleSuccess);
+    _razorpay.on(Razorpay.EVENT_PAYMENT_ERROR, _handleError);
+    _razorpay.on(Razorpay.EVENT_EXTERNAL_WALLET, _handleExternalWallet);
+  }
+
+  void openPayment({
+    required double amount,
+    required String email,
+    required String phone,
+    required String eventName,
+    String? orderId,
+  }) {
+    final options = <String, dynamic>{
+      'key': _testKey,
+      'amount': (amount * 100).toInt(), // paise
+      'currency': 'INR',
+      'name': 'Eventify',
+      'description': 'Ticket: $eventName',
+      'prefill': {
+        'email': email,
+        'contact': phone,
+      },
+      'theme': {'color': '#0B63D6'},
+    };
+    if (orderId != null) options['order_id'] = orderId;
+
+    if (kDebugMode) debugPrint('PaymentService: opening Razorpay with amount=${amount * 100} paise');
+    _razorpay.open(options);
+  }
+
+  void _handleSuccess(PaymentSuccessResponse res) => onSuccess?.call(res);
+  void _handleError(PaymentFailureResponse res) => onError?.call(res);
+  void _handleExternalWallet(ExternalWalletResponse res) => onExternalWallet?.call(res);
+
+  void dispose() {
+    _razorpay.clear();
+  }
+}