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/Filters/ValidationFilter.cs

@@ -0,0 +1,20 @@
+using FluentValidation;
+
+namespace Musify.Api.Filters;
+
+public sealed class ValidationFilter<T>(IValidator<T> validator) : IEndpointFilter where T : class
+{
+    public async ValueTask<object?> InvokeAsync(EndpointFilterInvocationContext context, EndpointFilterDelegate next)
+    {
+        var argument = context.Arguments.OfType<T>().FirstOrDefault();
+
+        if (argument is null)
+            return await next(context);
+
+        var result = await validator.ValidateAsync(argument, context.HttpContext.RequestAborted);
+
+        return result.IsValid
+            ? await next(context)
+            : Results.ValidationProblem(result.ToDictionary());
+    }
+}