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/models/invite_preview.dart

@@ -0,0 +1,28 @@
+class InvitePreview {
+  const InvitePreview({
+    required this.valid,
+    required this.email,
+    required this.teamName,
+    this.clubName,
+    this.teamId,
+    this.error,
+  });
+
+  final bool valid;
+  final String email;
+  final String teamName;
+  final String? clubName;
+  final String? teamId;
+  final String? error;
+
+  factory InvitePreview.fromJson(Map<String, dynamic> json) {
+    return InvitePreview(
+      valid: json['valid'] as bool? ?? false,
+      email: json['email'] as String? ?? '',
+      teamName: json['team_name'] as String? ?? '',
+      clubName: json['club_name'] as String?,
+      teamId: json['team_id'] as String?,
+      error: json['error'] as String?,
+    );
+  }
+}