upsertDeviceToken method
Implementation
Future<void> upsertDeviceToken(String fcmToken, String platform) async {
final userId = _client.auth.currentUser?.id;
if (userId == null) return;
await _client.from('device_tokens').upsert({
'user_id': userId,
'fcm_token': fcmToken,
'platform': platform,
'updated_at': DateTime.now().toIso8601String(),
}, onConflict: 'user_id,fcm_token');
}