fix: prevent re-download of tracks converted to a different format

When a file is converted externally (e.g. FLAC to OPUS), the
orphan cleanup would delete the history entry because the original
path no longer exists. Now it checks for sibling files with other
audio extensions and updates the stored path instead of deleting.

Also add extension-stripped keys to path_match_keys so that
paths differing only by audio extension still match during local
library scan exclusion and queue deduplication.

lib/utils/path_match_keys.dart

@@ -8,6 +8,33 @@ const _androidStoragePathAliases = <String>[
   '/mnt/sdcard',
 ];
 
+/// Audio file extensions that the app commonly produces or converts between.
+/// Used to generate extension-stripped match keys so that a file converted from
+/// one format to another (e.g. .flac → .opus) is still recognised as the same
+/// track.
+const _audioExtensions = <String>[
+  '.flac',
+  '.m4a',
+  '.mp3',
+  '.opus',
+  '.ogg',
+  '.wav',
+  '.aac',
+];
+
+/// Strips a trailing audio extension from [path] if present.
+/// Returns the path without extension, or `null` if no known audio extension
+/// was found.
+String? _stripAudioExtension(String path) {
+  final lower = path.toLowerCase();
+  for (final ext in _audioExtensions) {
+    if (lower.endsWith(ext)) {
+      return path.substring(0, path.length - ext.length);
+    }
+  }
+  return null;
+}
+
 Set<String> buildPathMatchKeys(String? filePath) {
   final raw = filePath?.trim() ?? '';
   if (raw.isEmpty) return const {};
@@ -79,6 +106,18 @@ Set<String> buildPathMatchKeys(String? filePath) {
   }
 
   addNormalized(cleaned);
+
+  // Add extension-stripped variants so that a file converted from one audio
+  // format to another (e.g. Song.flac → Song.opus) still matches.
+  final extensionStrippedKeys = <String>{};
+  for (final key in keys) {
+    final stripped = _stripAudioExtension(key);
+    if (stripped != null && stripped.isNotEmpty) {
+      extensionStrippedKeys.add(stripped);
+    }
+  }
+  keys.addAll(extensionStrippedKeys);
+
   return keys;
 }