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/auth/services/auth_service.dart

@@ -1,10 +1,12 @@
 // lib/features/auth/services/auth_service.dart
 import 'package:flutter/foundation.dart' show kDebugMode, debugPrint;
 import 'package:shared_preferences/shared_preferences.dart';
+import 'package:google_sign_in/google_sign_in.dart';
 import '../../../core/api/api_client.dart';
 import '../../../core/api/api_endpoints.dart';
 import '../../../core/auth/auth_guard.dart';
 import '../../../core/storage/token_storage.dart';
+import '../../../core/analytics/posthog_service.dart';
 import '../models/user_model.dart';
 
 class AuthService {
@@ -58,6 +60,12 @@ class AuthService {
       // Save phone if provided (optional)
       if (res['phone_number'] != null) await prefs.setString('phone_number', res['phone_number'].toString());
 
+      PostHogService.instance.identify(savedEmail, properties: {
+        'username': displayCandidate,
+        'login_method': 'email',
+      });
+      PostHogService.instance.capture('user_logged_in');
+
       return UserModel.fromJson(res);
     } catch (e) {
       if (kDebugMode) debugPrint('AuthService.login error: $e');
@@ -130,6 +138,54 @@ class AuthService {
     }
   }
 
+  /// GOOGLE OAUTH LOGIN → returns UserModel
+  Future<UserModel> googleLogin() async {
+    try {
+      final googleSignIn = GoogleSignIn(scopes: ['email']);
+      final account = await googleSignIn.signIn();
+      if (account == null) throw Exception('Google sign-in cancelled');
+
+      final auth = await account.authentication;
+      final idToken = auth.idToken;
+      if (idToken == null) throw Exception('Failed to get Google ID token');
+
+      final res = await _api.post(
+        ApiEndpoints.googleLogin,
+        body: {'id_token': idToken},
+        requiresAuth: false,
+      );
+
+      final token = res['token'];
+      if (token == null) throw Exception('Token missing from response');
+
+      final serverEmail = (res['email'] as String?) ?? account.email;
+      final displayName = (res['username'] as String?) ?? account.displayName ?? serverEmail;
+
+      AuthGuard.setGuest(false);
+      await TokenStorage.saveToken(token.toString(), serverEmail);
+
+      final prefs = await SharedPreferences.getInstance();
+      await prefs.setString('current_email', serverEmail);
+      await prefs.setString('email', serverEmail);
+      final perKey = 'display_name_$serverEmail';
+      if ((prefs.getString(perKey) ?? '').isEmpty) {
+        await prefs.setString(perKey, displayName);
+      }
+      if (res['phone_number'] != null) await prefs.setString('phone_number', res['phone_number'].toString());
+
+      PostHogService.instance.identify(serverEmail, properties: {
+        'username': displayName,
+        'login_method': 'google',
+      });
+      PostHogService.instance.capture('user_logged_in');
+
+      return UserModel.fromJson(res);
+    } catch (e) {
+      if (kDebugMode) debugPrint('AuthService.googleLogin error: $e');
+      rethrow;
+    }
+  }
+
   /// Logout – clear auth token and current_email (keep per-account display_name entries so they persist)
   Future<void> logout() async {
     try {
@@ -141,6 +197,8 @@ class AuthService {
       // Also remove canonical 'email' pointing to current user
       await prefs.remove('email');
       // Do not delete display_name_<email> entries — they are per-account and should remain on device.
+      PostHogService.instance.capture('user_logged_out');
+      PostHogService.instance.reset();
     } catch (e) {
       if (kDebugMode) debugPrint('AuthService.logout warning: $e');
     }