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/features/reviews/widgets/review_section.dart

@@ -6,6 +6,7 @@ import '../../../widgets/bouncing_loader.dart';
 import '../../../core/utils/error_utils.dart';
 import '../models/review_models.dart';
 import '../services/review_service.dart';
+import '../../../core/analytics/posthog_service.dart';
 import 'review_summary.dart';
 import 'review_form.dart';
 import 'review_card.dart';
@@ -82,6 +83,10 @@ class _ReviewSectionState extends State<ReviewSection> {
 
   Future<void> _handleSubmit(int rating, String? comment) async {
     await _service.submitReview(widget.eventId, rating, comment);
+    PostHogService.instance.capture('review_submitted', properties: {
+      'event_id': widget.eventId,
+      'rating': rating,
+    });
     await _loadReviews(); // Refresh to get updated stats + review list
   }