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/auth/providers/auth_provider.dart

@@ -59,6 +59,19 @@ class AuthProvider extends ChangeNotifier {
     }
   }
 
+  /// Google OAuth login.
+  Future<void> googleLogin() async {
+    _loading = true;
+    notifyListeners();
+    try {
+      final user = await _authService.googleLogin();
+      _user = user;
+    } finally {
+      _loading = false;
+      notifyListeners();
+    }
+  }
+
   Future<void> logout() async {
     await _authService.logout();
     _user = null;