Fix failedList removal in deleteAsync and narrow task exception handler

- deleteAsync removed a failedList entry from the downloadQueue (a no-op:
  failed items were already polled out of the queue), leaving stale FAILED
  state for deleted tracks. Remove the entry from failedList instead.
- The DownloadTask CoroutineExceptionHandler deleted the complete and
  pinned files on any uncaught exception. Those files may already hold a
  finalized download (exceptions can occur in post-rename steps such as
  the catalog write), so this destroyed finished data. Only the partial
  file is deleted now: retries do not append to a possibly untrustworthy
  stream, while finalized downloads and cancelled (resumable) downloads
  are left untouched.

ultrasonic/src/main/kotlin/org/moire/ultrasonic/service/DownloadService.kt

@@ -423,7 +423,7 @@ class DownloadService :
         private suspend fun deleteAsync(track: Track) {
             withContext(Dispatchers.IO) {
                 downloadQueue.get(track.id)?.let { downloadQueue.remove(it) }
-                failedList[track.id]?.let { downloadQueue.remove(it) }
+                failedList.remove(track.id)
                 cancelDownload(track)
 
                 Storage.delete(track.getPartialFile())

ultrasonic/src/main/kotlin/org/moire/ultrasonic/service/DownloadTask.kt

@@ -206,8 +206,14 @@ class DownloadTask(
             exception
         ->
         Timber.w(exception, "Exception in DownloadTask ${downloadTrack.pinnedFile}")
-        Storage.delete(downloadTrack.completeFile)
-        Storage.delete(downloadTrack.pinnedFile)
+        // Only ever touch the partial file here: the complete/pinned files may hold a
+        // download that has already been finalized (the exception may have happened
+        // during a post-rename step such as the catalog write), so deleting them would
+        // destroy finished data. The partial file is removed so that a retry does not
+        // append to a possibly untrustworthy stream; pure cancellation keeps the partial
+        // file, because the exception handler is not invoked for CancellationException
+        // and cancelled downloads should remain resumable.
+        Storage.delete(downloadTrack.partialFile)
     }
 
     fun start() {