feat: add subsonic proxy architecture with external music services

- Add MusicModels (Song, Artist, Album, SearchResult, DownloadInfo)
- Add IMusicMetadataService and DeezerMetadataService for external search
- Add IDownloadService and DeezspotDownloadService for downloads
- Add LocalLibraryService for managing downloaded songs cache
- Add custom endpoints: search3, stream, getSong, getAlbum, getCoverArt
- Configure dependency injection for all services

octo-fiesta/appsettings.json

@@ -5,5 +5,14 @@
       "Microsoft.AspNetCore": "Warning"
     }
   },
-  "AllowedHosts": "*"
+  "AllowedHosts": "*",
+  "Subsonic": {
+    "Url": "http://localhost:4533"
+  },
+  "Library": {
+    "DownloadPath": "./downloads"
+  },
+  "Deezspot": {
+    "ExecutablePath": ""
+  }
 }