feat: PostHog analytics wiring across all key screens

- Commit untracked posthog_service.dart (fire-and-forget HTTP client,
  EU data residency, already used by auth for identify/reset)
- screen() calls: Home, Contribute, Profile, EventDetail (with event_id)
- capture('event_tapped') on hero carousel card tap (source: hero_carousel)
- capture('book_now_tapped') in _navigateToCheckout (event_id + name)
- capture('review_submitted') in _handleSubmit (event_id + rating)
- Covers all 4 expansion items from security audit finding 8.2

lib/screens/home_screen.dart

@@ -24,6 +24,7 @@ import '../features/gamification/providers/gamification_provider.dart';
 import '../features/notifications/widgets/notification_bell.dart';
 import '../features/notifications/providers/notification_provider.dart';
 import '../widgets/skeleton_loader.dart';
+import '../core/analytics/posthog_service.dart';
 
 class HomeScreen extends StatefulWidget {
   const HomeScreen({Key? key}) : super(key: key);
@@ -62,6 +63,7 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
     _heroPageNotifier = ValueNotifier(0);
     _loadUserDataAndEvents();
     _startAutoScroll();
+    PostHogService.instance.screen('Home');
   }
 
   @override
@@ -1382,6 +1384,11 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
     return GestureDetector(
       onTap: () {
         if (event.id != null) {
+          PostHogService.instance.capture('event_tapped', properties: {
+            'event_id': event.id,
+            'title': event.title ?? event.name ?? '',
+            'source': 'hero_carousel',
+          });
           Navigator.push(context,
               MaterialPageRoute(builder: (_) => LearnMoreScreen(eventId: event.id, initialEvent: event)));
         }