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.StreamingGateway/Program.cs

@@ -0,0 +1,42 @@
+using Musify.StreamingGateway.Authentication;
+using Musify.StreamingGateway.Configuration;
+using Musify.StreamingGateway.Middleware;
+
+var builder = WebApplication.CreateBuilder();
+
+builder.Services
+    .AddOptions<StreamTicketValidationOptions>()
+    .Bind(builder.Configuration.GetRequiredSection(StreamTicketValidationOptions.SectionName))
+    .ValidateDataAnnotations()
+    .ValidateOnStart();
+
+builder.Services.AddSingleton<TicketValidator>();
+
+const string corsPolicy = "media-clients";
+var allowedOrigins = builder.Configuration.GetSection("Cors:AllowedOrigins").Get<string[]>() ?? [];
+builder.Services.AddCors(options => options.AddPolicy(corsPolicy, policy =>
+{
+    if (builder.Environment.IsDevelopment())
+        policy.AllowAnyOrigin();
+    else
+        policy.WithOrigins(allowedOrigins);
+
+    policy.AllowAnyHeader()
+        .WithMethods("GET", "HEAD")
+        .WithExposedHeaders("Content-Range", "Accept-Ranges", "Content-Length");
+} ));
+
+builder.Services
+    .AddReverseProxy()
+    .LoadFromConfig(builder.Configuration.GetSection("ReverseProxy"));
+
+var app = builder.Build();
+
+app.MapGet("/health", () => Results.Ok("healthy"))
+    .WithSummary("Health Check Endpoint");
+
+app.UseCors(corsPolicy);
+app.UseMiddleware<TicketValidationMiddleware>();
+app.MapReverseProxy();
+
+app.Run();