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/Users/CreateUserRequestValidator.cs

@@ -0,0 +1,25 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Users;
+
+namespace Musify.Api.Validators.Users;
+
+public sealed class CreateUserRequestValidator : AbstractValidator<CreateUserRequest>
+{
+    public CreateUserRequestValidator()
+    {
+        RuleFor(x => x.Id)
+            .NotEmpty();
+
+        RuleFor(x => x.Name)
+            .NotEmpty()
+            .MaximumLength(48);
+
+        RuleFor(x => x.FirstName)
+            .MaximumLength(48)
+            .When(x => x.FirstName is not null);
+
+        RuleFor(x => x.SecondName)
+            .MaximumLength(48)
+            .When(x => x.SecondName is not null);
+    }
+}