refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

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());
+    }
+}