chore: group .NET into backend/ and drop non-.NET test artifacts

- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
  into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore

Build from backend/Musify.slnx: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Application/Extensions/ResultExtensions.cs

@@ -0,0 +1,25 @@
+using Ardalis.Result;
+
+namespace Musify.Application.Extensions;
+
+public static class ResultExtensions
+{
+    public static Result<TOut> As<TIn, TOut>(this Result<TIn> result)
+    {
+        return result.Status switch
+        {
+            ResultStatus.Ok or
+            ResultStatus.Created or
+            ResultStatus.NoContent   => throw new InvalidOperationException($"Cannot convert a successful result ( {result.Status} ) to another type. Use Map or Bind instead."),
+            ResultStatus.NotFound    => Result<TOut>.NotFound(result.Errors.ToArray()),
+            ResultStatus.Forbidden   => Result<TOut>.Forbidden(result.Errors.ToArray()),
+            ResultStatus.Unauthorized => Result<TOut>.Unauthorized(result.Errors.ToArray()),
+            ResultStatus.Invalid     => Result<TOut>.Invalid(result.ValidationErrors.ToArray()),
+            ResultStatus.Conflict    => Result<TOut>.Conflict(result.Errors.ToArray()),
+            ResultStatus.CriticalError => Result<TOut>.CriticalError(result.Errors.ToArray()),
+            ResultStatus.Unavailable => Result<TOut>.Unavailable(result.Errors.ToArray()),
+            ResultStatus.Error       => Result<TOut>.Error(string.Join(";", result.Errors)),
+            _                        => Result<TOut>.Error(string.Join(";", result.Errors))
+        };
+    }
+}