Billing upgrade/downgrade, inviti in app e diretta YouTube da mobile.

Upgrade Stripe con addebito immediato; downgrade programmato a fine periodo.
UX billing più sobria con box info; icone piani. API inviti e OAuth YouTube
per staff trasmissione; deep link join; scelta partita programmata o nuova.

Co-authored-by: Cursor <cursoragent@cursor.com>

mobile/lib/shared/api_client.dart

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../core/config.dart';
 import '../providers/auth_provider.dart';
+import 'models/invite_preview.dart';
 import 'models/match.dart';
 import 'models/recording_item.dart';
 import 'models/stream_session.dart';
@@ -86,6 +87,24 @@ class ApiClient {
         .toList();
   }
 
+  Future<InvitePreview> fetchInvitePreview(String token) async {
+    final response = await _dio.get<Map<String, dynamic>>('/invitations/$token');
+    return InvitePreview.fromJson(response.data!);
+  }
+
+  Future<String> acceptInvitation(String token) async {
+    final response = await _dio.post<Map<String, dynamic>>('/invitations/$token/accept');
+    return response.data!['message'] as String? ?? 'Invito accettato';
+  }
+
+  Future<String> youtubeAuthorizeUrl(String teamId, {bool returnApp = true}) async {
+    final response = await _dio.get<Map<String, dynamic>>(
+      '/teams/$teamId/youtube/authorize',
+      queryParameters: returnApp ? {'return_app': '1'} : null,
+    );
+    return response.data!['authorization_url'] as String;
+  }
+
   Future<List<RecordingItem>> fetchRecordings(String teamId) async {
     final response = await _dio.get<List<dynamic>>('/teams/$teamId/recordings');
     return response.data!