fix: address Copilot PR review findings

- Register DeezerDownloadService and DeezerMetadataService as Singleton
  to properly share state across requests (rate limiting, download tracking)
- Fix race condition in LocalLibraryService.LoadMappingsAsync with
  double-check locking pattern
- Dispose HttpRequestMessage objects to prevent memory leaks (4 occurrences)
- Handle fire-and-forget TriggerLibraryScanAsync with proper error logging
- Replace Console.WriteLine with ILogger in SubsonicController
- Fix while loop in DownloadSongAsync to refresh activeDownload state
- Use modern C# range operator syntax for Substring calls
- Clean up appsettings.json template (remove private IP, clear ARL token)
- Add documentation comment for Blowfish decryption key
- Add downloads directory to gitignore

.gitignore

@@ -1,4 +1,4 @@
-## A streamlined .gitignore for modern .NET projects
+## A streamlined .gitignore for modern .NET projects
 ## including temporary files, build results, and
 ## files generated by popular .NET tools. If you are
 ## developing with Visual Studio, the VS .gitignore
@@ -68,4 +68,7 @@ obj/
 # Autres fichiers temporaires
 *.log
 
-/.env
+/.env
+
+# Downloaded music files
+octo-fiesta/downloads/