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/Validators/PlayLists/RequestPlayListPictureUploadRequestValidator.cs

@@ -0,0 +1,20 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class RequestPlayListPictureUploadRequestValidator : AbstractValidator<RequestPlayListPictureUploadRequest>
+{
+    public RequestPlayListPictureUploadRequestValidator()
+    {
+        RuleFor(x => x.FileType)
+            .NotEmpty();
+
+        RuleFor(x => x.ContentType)
+            .NotEmpty();
+
+        RuleFor(x => x.ExpectedSizeBytes)
+            .GreaterThan(0)
+            .When(x => x.ExpectedSizeBytes.HasValue);
+    }
+}