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.Api/Program.cs

@@ -0,0 +1,37 @@
+using Musify.Infrastructure.Observability;
+using Scalar.AspNetCore;
+using Musify.Api.Endpoints;
+using Musify.Api.Extensions;
+
+var builder = WebApplication.CreateBuilder();
+
+const string devCorsPolicy = "dev-cors";
+
+builder.Services
+    .AddApplicationServices(builder.Configuration)
+    .AddObservability(builder.Configuration);
+
+if (builder.Environment.IsDevelopment())
+{
+    builder.Services.AddCors(options => options.AddPolicy(devCorsPolicy, policy =>
+        policy.AllowAnyOrigin().AllowAnyHeader().AllowAnyMethod()));
+}
+
+var app = builder.Build();
+
+if (app.Environment.IsDevelopment())
+{
+    app.UseCors(devCorsPolicy);
+    app.MapOpenApi();
+    app.MapScalarApiReference();
+}
+
+app.UseAuthentication();
+app.UseAuthorization();
+
+app
+    .MapPlayListEndpoints()
+    .MapTrackEndpoints()
+    .MapUserEndpoints();
+
+app.Run();