fix: use correct 'subsonic-response' key format in JSON responses

ASP.NET Core serializes anonymous objects with camelCase, producing
'subsonicResponse' instead of 'subsonic-response'. Fixed by using
Dictionary with explicit key names for all JSON responses.

package-lock.json

@@ -0,0 +1,6 @@
+{
+  "name": "octo-fiesta",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {}
+}