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

octo-fiesta/appsettings.json

@@ -7,7 +7,7 @@
   },
   "AllowedHosts": "*",
   "Subsonic": {
-    "Url": "http://192.168.1.12:4533"
+    "Url": "http://localhost:4533"
   },
   "Library": {
     "DownloadPath": "./downloads"