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

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core/analytics/posthog_service.dart

@@ -0,0 +1,94 @@
+// lib/core/analytics/posthog_service.dart
+import 'dart:convert';
+import 'dart:io' show Platform;
+import 'package:flutter/foundation.dart';
+import 'package:http/http.dart' as http;
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// Lightweight PostHog analytics client using the HTTP API.
+/// Works with Dart 2.x (no posthog_flutter SDK needed).
+class PostHogService {
+  static const String _apiKey = 'phc_xXxn0COAwWRj3AU7fspsTuesCIK0aBGXb3zaIIJRgZA';
+  static const String _host = 'https://eu.i.posthog.com';
+  static const String _distinctIdKey = 'posthog_distinct_id';
+
+  static PostHogService? _instance;
+  String? _distinctId;
+
+  PostHogService._();
+
+  static PostHogService get instance {
+    _instance ??= PostHogService._();
+    return _instance!;
+  }
+
+  /// Initialize and load or generate a distinct ID.
+  Future<void> init() async {
+    final prefs = await SharedPreferences.getInstance();
+    _distinctId = prefs.getString(_distinctIdKey);
+    if (_distinctId == null) {
+      _distinctId = DateTime.now().millisecondsSinceEpoch.toRadixString(36) +
+          UniqueKey().toString().hashCode.toRadixString(36);
+      await prefs.setString(_distinctIdKey, _distinctId!);
+    }
+  }
+
+  /// Identify a user (call after login).
+  void identify(String userId, {Map<String, dynamic>? properties}) {
+    _distinctId = userId;
+    SharedPreferences.getInstance().then((prefs) {
+      prefs.setString(_distinctIdKey, userId);
+    });
+    _send('identify', {
+      'distinct_id': userId,
+      if (properties != null) '\$set': properties,
+    });
+  }
+
+  /// Capture a custom event.
+  void capture(String event, {Map<String, dynamic>? properties}) {
+    _send('capture', {
+      'event': event,
+      'distinct_id': _distinctId ?? 'anonymous',
+      'properties': {
+        ...?properties,
+        '\$lib': 'flutter',
+        '\$lib_version': '1.0.0',
+      },
+    });
+  }
+
+  /// Capture a screen view.
+  void screen(String screenName, {Map<String, dynamic>? properties}) {
+    capture('\$screen', properties: {
+      '\$screen_name': screenName,
+      ...?properties,
+    });
+  }
+
+  /// Reset identity (call on logout).
+  void reset() {
+    _distinctId = null;
+    SharedPreferences.getInstance().then((prefs) {
+      prefs.remove(_distinctIdKey);
+    });
+  }
+
+  /// Send event to PostHog API (fire-and-forget).
+  void _send(String endpoint, Map<String, dynamic> body) {
+    final payload = {
+      'api_key': _apiKey,
+      'timestamp': DateTime.now().toUtc().toIso8601String(),
+      ...body,
+    };
+
+    // Fire and forget — don't block the UI
+    http.post(
+      Uri.parse('$_host/$endpoint/'),
+      headers: {'Content-Type': 'application/json'},
+      body: jsonEncode(payload),
+    ).catchError((e) {
+      if (kDebugMode) debugPrint('PostHog error: $e');
+    });
+  }
+}

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
   }
 

lib/screens/contribute_screen.dart

@@ -21,6 +21,7 @@ import '../widgets/landscape_section_header.dart';
 import '../widgets/tier_avatar_ring.dart';
 import '../features/share/share_rank_card.dart';
 import 'contributor_profile_screen.dart';
+import '../core/analytics/posthog_service.dart';
 
 // ─────────────────────────────────────────────────────────────────────────────
 // Tier colour map
@@ -102,6 +103,7 @@ class _ContributeScreenState extends State<ContributeScreen>
   @override
   void initState() {
     super.initState();
+    PostHogService.instance.screen('Contribute');
     WidgetsBinding.instance.addPostFrameCallback((_) {
       context.read<GamificationProvider>().loadAll();
     });

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)));
         }

lib/screens/learn_more_screen.dart

@@ -18,6 +18,7 @@ import '../features/reviews/widgets/review_section.dart';
 import '../widgets/tier_avatar_ring.dart';
 import 'contributor_profile_screen.dart';
 import 'checkout_screen.dart';
+import '../core/analytics/posthog_service.dart';
 
 class LearnMoreScreen extends StatefulWidget {
   final int eventId;
@@ -37,6 +38,10 @@ class _LearnMoreScreenState extends State<LearnMoreScreen> with SingleTickerProv
   void _navigateToCheckout() {
     if (!AuthGuard.requireLogin(context, reason: 'Sign in to book this event.')) return;
     if (_event == null) return;
+    PostHogService.instance.capture('book_now_tapped', properties: {
+      'event_id': _event!.id,
+      'event_name': _event!.name ?? _event!.title ?? '',
+    });
     Navigator.of(context).push(MaterialPageRoute(
       builder: (_) => CheckoutScreen(
         eventId: _event!.id,
@@ -71,6 +76,7 @@ class _LearnMoreScreenState extends State<LearnMoreScreen> with SingleTickerProv
   @override
   void initState() {
     super.initState();
+    PostHogService.instance.screen('EventDetail', properties: {'event_id': widget.eventId});
     _fadeController = AnimationController(vsync: this, duration: const Duration(milliseconds: 350));
     _fade = CurvedAnimation(parent: _fadeController, curve: Curves.easeIn);
     _pageNotifier = ValueNotifier(0);

lib/screens/profile_screen.dart

@@ -24,6 +24,7 @@ import '../core/app_decoration.dart';
 import '../core/constants.dart';
 import '../widgets/landscape_section_header.dart';
 import '../features/share/share_rank_card.dart';
+import '../core/analytics/posthog_service.dart';
 
 class ProfileScreen extends StatefulWidget {
   const ProfileScreen({Key? key}) : super(key: key);
@@ -99,6 +100,7 @@ class _ProfileScreenState extends State<ProfileScreen>
   @override
   void initState() {
     super.initState();
+    PostHogService.instance.screen('Profile');
 
     // Animation controller for EXP bar + stat counters
     _animController = AnimationController(