fix: use identifier as fallback key for EPG channels

lib/models/livetv_channel.dart

@@ -34,7 +34,7 @@ class LiveTvChannel {
 
   factory LiveTvChannel.fromJson(Map<String, dynamic> json) {
     return LiveTvChannel(
-      key: json['key'] as String? ?? json['ratingKey'] as String? ?? '',
+      key: json['key'] as String? ?? json['ratingKey'] as String? ?? json['identifier'] as String? ?? json['channelIdentifier'] as String? ?? '',
       identifier: json['identifier'] as String? ?? json['channelIdentifier'] as String?,
       callSign: json['callSign'] as String?,
       title: json['title'] as String? ?? json['callSign'] as String?,

lib/services/plex_client.dart

@@ -2037,6 +2037,7 @@ class PlexClient {
             .where((ch) => ch.key.isNotEmpty)
             .toList();
       }
+      appLogger.d('EPG channels: container keys=${container?.keys.toList()}, size=${container?['size']}');
       return [];
     }, 'Failed to get EPG channels');
   }