fix: prevent duplicate downloads and lock release bug

- Track lock state to prevent double-release in finally block
- Fixes exception when download is already in progress
- Prevents duplicate file downloads with (1), (2), (3) suffixes
- Ensures proper lock management during concurrent download requests

allstarr/Services/Common/BaseDownloadService.cs

@@ -264,6 +264,7 @@ public abstract class BaseDownloadService : IDownloadService
         
         // Acquire lock BEFORE checking existence to prevent race conditions with concurrent requests
         await DownloadLock.WaitAsync(cancellationToken);
+        var lockHeld = true;
         
         try
         {
@@ -288,6 +289,7 @@ public abstract class BaseDownloadService : IDownloadService
                 Logger.LogDebug("Download already in progress for {SongId}, waiting for completion...", songId);
                 // Release lock while waiting
                 DownloadLock.Release();
+                lockHeld = false;
                 
                 // Wait for download to complete, checking every 100ms (faster than 500ms)
                 // Also respect cancellation token so client timeouts are handled immediately
@@ -443,10 +445,13 @@ public abstract class BaseDownloadService : IDownloadService
             throw;
         }
         finally
+        {
+            if (lockHeld)
             {
                 DownloadLock.Release();
             }
         }
+    }
     
     protected async Task DownloadRemainingAlbumTracksAsync(string albumExternalId, string excludeTrackExternalId)
     {