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/notifications/models/notification_model.dart

@@ -0,0 +1,33 @@
+// lib/features/notifications/models/notification_model.dart
+
+class NotificationModel {
+  final int id;
+  final String title;
+  final String message;
+  final String type; // event, promo, system, booking
+  bool isRead;
+  final DateTime createdAt;
+  final String? actionUrl;
+
+  NotificationModel({
+    required this.id,
+    required this.title,
+    required this.message,
+    this.type = 'system',
+    this.isRead = false,
+    required this.createdAt,
+    this.actionUrl,
+  });
+
+  factory NotificationModel.fromJson(Map<String, dynamic> json) {
+    return NotificationModel(
+      id: (json['id'] as num?)?.toInt() ?? 0,
+      title: json['title'] as String? ?? '',
+      message: json['message'] as String? ?? '',
+      type: json['notification_type'] as String? ?? json['type'] as String? ?? 'system',
+      isRead: (json['is_read'] as bool?) ?? false,
+      createdAt: DateTime.tryParse(json['created_at'] as String? ?? '') ?? DateTime.now(),
+      actionUrl: json['action_url'] as String?,
+    );
+  }
+}