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/Tracks/CreateTrackRequestValidator.cs

@@ -0,0 +1,20 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Tracks;
+
+namespace Musify.Api.Validators.Tracks;
+
+public sealed class CreateTrackRequestValidator : AbstractValidator<CreateTrackRequest>
+{
+    public CreateTrackRequestValidator()
+    {
+        RuleFor(x => x.Title)
+            .NotEmpty()
+            .MaximumLength(50);
+
+        RuleFor(x => x.PictureIntentId)
+            .NotEmpty();
+
+        RuleFor(x => x.AudioIntentId)
+            .NotEmpty();
+    }
+}