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/Authentication/AuthenticationExtensions.cs

@@ -0,0 +1,31 @@
+using Microsoft.AspNetCore.Authentication.JwtBearer;
+using Microsoft.Extensions.Options;
+
+namespace Musify.Api.Authentication;
+
+public static class AuthenticationExtensions
+{
+    public static IServiceCollection AddAuthenticationConfiguration(
+        this IServiceCollection services,
+        IConfiguration configuration)
+    {
+        services
+            .AddOptionsWithValidateOnStart<AuthenticationConfiguration>()
+            .Bind(configuration.GetRequiredSection(AuthenticationConfiguration.SectionName))
+            .ValidateDataAnnotations();
+
+        services.AddSingleton(serviceProvider =>
+            serviceProvider.GetRequiredService<IOptions<AuthenticationConfiguration>>().Value);
+
+        services.AddScoped<JwtBearerEventsHandler>();
+        services.ConfigureOptions<JwtBearerOptionsSetup>();
+
+        services
+            .AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
+            .AddJwtBearer();
+
+        services.AddAuthorization();
+
+        return services;
+    }
+}