feat(download): add {playlist_position} filename placeholder

Add a playlist position token usable in filename templates, plumbed from the playlist screen through the download queue to the Go backend. Auto-prefix playlist downloads with the position when the template lacks the token. Includes Go filename test.

lib/models/download_item.dart

@@ -37,6 +37,7 @@ class DownloadItem {
   final DateTime createdAt;
   final String? qualityOverride; // Override quality for this specific download
   final String? playlistName; // Playlist context for folder organization
+  final int? playlistPosition; // 1-based position in the source playlist
 
   const DownloadItem({
     required this.id,
@@ -53,6 +54,7 @@ class DownloadItem {
     required this.createdAt,
     this.qualityOverride,
     this.playlistName,
+    this.playlistPosition,
   });
 
   DownloadItem copyWith({
@@ -70,6 +72,7 @@ class DownloadItem {
     DateTime? createdAt,
     String? qualityOverride,
     String? playlistName,
+    int? playlistPosition,
   }) {
     return DownloadItem(
       id: id ?? this.id,
@@ -86,6 +89,7 @@ class DownloadItem {
       createdAt: createdAt ?? this.createdAt,
       qualityOverride: qualityOverride ?? this.qualityOverride,
       playlistName: playlistName ?? this.playlistName,
+      playlistPosition: playlistPosition ?? this.playlistPosition,
     );
   }
 

lib/models/download_item.g.dart

@@ -23,6 +23,7 @@ DownloadItem _$DownloadItemFromJson(Map<String, dynamic> json) => DownloadItem(
   createdAt: DateTime.parse(json['createdAt'] as String),
   qualityOverride: json['qualityOverride'] as String?,
   playlistName: json['playlistName'] as String?,
+  playlistPosition: (json['playlistPosition'] as num?)?.toInt(),
 );
 
 Map<String, dynamic> _$DownloadItemToJson(DownloadItem instance) =>
@@ -41,6 +42,7 @@ Map<String, dynamic> _$DownloadItemToJson(DownloadItem instance) =>
       'createdAt': instance.createdAt.toIso8601String(),
       'qualityOverride': instance.qualityOverride,
       'playlistName': instance.playlistName,
+      'playlistPosition': instance.playlistPosition,
     };
 
 const _$DownloadStatusEnumMap = {