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.

backend/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();