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

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Musify.Infrastructure/MassTransit/DependencyInjection/MassTransitDependencyInjection.Options.cs

@@ -0,0 +1,24 @@
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Options;
+
+namespace Musify.Infrastructure.MassTransit
+{
+    public static partial class MassTransitDependencyInjection
+    {
+        static void RegisterValidatedOptions<TOptions>(
+            IServiceCollection services,
+            IConfiguration configuration,
+            string sectionName)
+            where TOptions : class
+        {
+            services
+                .AddOptionsWithValidateOnStart<TOptions>()
+                .Bind(configuration.GetRequiredSection(sectionName))
+                .ValidateDataAnnotations();
+
+            services.AddSingleton(serviceProvider =>
+                serviceProvider.GetRequiredService<IOptions<TOptions>>().Value);
+        }
+    }
+}