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.Api/Extensions/ResultHttpExtensions.cs

@@ -0,0 +1,36 @@
+using Ardalis.Result;
+using IResult = Microsoft.AspNetCore.Http.IResult;
+
+namespace Musify.Api.Extensions;
+
+public static class ResultHttpExtensions
+{
+    public static IResult ToHttpResult(this Result result) =>
+        result.Status switch
+        {
+            ResultStatus.Ok or ResultStatus.NoContent => Results.NoContent(),
+            ResultStatus.NotFound => Results.NotFound(result.Errors.FirstOrDefault()),
+            ResultStatus.Invalid => Results.ValidationProblem(ToValidationDictionary(result.ValidationErrors)),
+            ResultStatus.Unauthorized => Results.Unauthorized(),
+            ResultStatus.Forbidden => Results.StatusCode(StatusCodes.Status403Forbidden),
+            ResultStatus.Conflict => Results.Conflict(result.Errors.FirstOrDefault()),
+            _ => Results.Problem(string.Join(", ", result.Errors))
+        };
+
+    public static IResult ToHttpResult<T>(this Result<T> result) =>
+        result.Status switch
+        {
+            ResultStatus.Ok => Results.Ok(result.Value),
+            ResultStatus.NotFound => Results.NotFound(result.Errors.FirstOrDefault()),
+            ResultStatus.Invalid => Results.ValidationProblem(ToValidationDictionary(result.ValidationErrors)),
+            ResultStatus.Unauthorized => Results.Unauthorized(),
+            ResultStatus.Forbidden => Results.StatusCode(StatusCodes.Status403Forbidden),
+            ResultStatus.Conflict => Results.Conflict(result.Errors.FirstOrDefault()),
+            _ => Results.Problem(string.Join(", ", result.Errors))
+        };
+
+    private static Dictionary<string, string[]> ToValidationDictionary(IEnumerable<ValidationError> errors) =>
+        errors
+            .GroupBy(e => string.IsNullOrEmpty(e.Identifier) ? "general" : e.Identifier)
+            .ToDictionary(g => g.Key, g => g.Select(e => e.ErrorMessage).ToArray());
+}