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

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/features/booking/providers/checkout_provider.dart

@@ -0,0 +1,147 @@
+// lib/features/booking/providers/checkout_provider.dart
+
+import 'package:flutter/foundation.dart';
+import '../../../core/utils/error_utils.dart';
+import '../models/booking_models.dart';
+import '../services/booking_service.dart';
+
+enum CheckoutStep { tickets, details, payment, confirmation }
+
+class CheckoutProvider extends ChangeNotifier {
+  final BookingService _service = BookingService();
+
+  // Event being booked
+  int? eventId;
+  String eventName = '';
+
+  // Step tracking
+  CheckoutStep currentStep = CheckoutStep.tickets;
+
+  // Ticket selection
+  List<TicketMetaModel> availableTickets = [];
+  List<CartItemModel> cart = [];
+
+  // Shipping
+  ShippingDetails? shippingDetails;
+
+  // Coupon
+  String? couponCode;
+
+  // Status
+  bool loading = false;
+  String? error;
+  String? paymentId;
+
+  /// Initialize checkout for an event.
+  Future<void> initForEvent(int eventId, String eventName) async {
+    this.eventId = eventId;
+    this.eventName = eventName;
+    currentStep = CheckoutStep.tickets;
+    cart = [];
+    shippingDetails = null;
+    couponCode = null;
+    paymentId = null;
+    error = null;
+    loading = true;
+    notifyListeners();
+
+    try {
+      availableTickets = await _service.getTicketMeta(eventId);
+    } catch (e) {
+      error = userFriendlyError(e);
+    } finally {
+      loading = false;
+      notifyListeners();
+    }
+  }
+
+  /// Add or update cart item.
+  void setTicketQuantity(TicketMetaModel ticket, int qty) {
+    cart.removeWhere((c) => c.ticket.id == ticket.id);
+    if (qty > 0) {
+      cart.add(CartItemModel(ticket: ticket, quantity: qty));
+    }
+    notifyListeners();
+  }
+
+  double get subtotal => cart.fold(0, (sum, item) => sum + item.subtotal);
+  double get total => subtotal; // expand with discount/tax later
+
+  bool get hasItems => cart.isNotEmpty;
+
+  /// Move to next step.
+  void nextStep() {
+    if (currentStep == CheckoutStep.tickets && hasItems) {
+      currentStep = CheckoutStep.details;
+    } else if (currentStep == CheckoutStep.details && shippingDetails != null) {
+      currentStep = CheckoutStep.payment;
+    }
+    notifyListeners();
+  }
+
+  /// Move to previous step.
+  void previousStep() {
+    if (currentStep == CheckoutStep.payment) {
+      currentStep = CheckoutStep.details;
+    } else if (currentStep == CheckoutStep.details) {
+      currentStep = CheckoutStep.tickets;
+    }
+    notifyListeners();
+  }
+
+  /// Set shipping details from form.
+  void setShipping(ShippingDetails details) {
+    shippingDetails = details;
+    notifyListeners();
+  }
+
+  /// Process checkout on backend.
+  Future<Map<String, dynamic>> processCheckout() async {
+    loading = true;
+    error = null;
+    notifyListeners();
+
+    try {
+      final tickets = cart.map((c) => {
+        'ticket_meta_id': c.ticket.id,
+        'quantity': c.quantity,
+      }).toList();
+
+      final res = await _service.processCheckout(
+        eventId: eventId!,
+        tickets: tickets,
+        shippingDetails: shippingDetails?.toJson() ?? {},
+        couponCode: couponCode,
+      );
+      return res;
+    } catch (e) {
+      error = userFriendlyError(e);
+      rethrow;
+    } finally {
+      loading = false;
+      notifyListeners();
+    }
+  }
+
+  /// Mark payment as complete.
+  void markPaymentSuccess(String id) {
+    paymentId = id;
+    currentStep = CheckoutStep.confirmation;
+    notifyListeners();
+  }
+
+  /// Reset checkout state.
+  void reset() {
+    eventId = null;
+    eventName = '';
+    currentStep = CheckoutStep.tickets;
+    availableTickets = [];
+    cart = [];
+    shippingDetails = null;
+    couponCode = null;
+    paymentId = null;
+    error = null;
+    loading = false;
+    notifyListeners();
+  }
+}