Reorganize backend to mirror Musify.slnx folder structure

Moves the six backend projects on disk into Core/ and Hosts/
subfolders, matching the solution folders already defined in
Musify.slnx, so the file explorer reflects the same layout as the
solution:

  backend/Core/Musify.Domain
  backend/Core/Musify.Application
  backend/Core/Musify.Infrastructure
  backend/Hosts/Musify.Api
  backend/Hosts/Musify.Worker
  backend/Hosts/Musify.StreamingGateway

Updates every reference to the old paths: ProjectReference entries in
Musify.Api.csproj and Musify.Worker.csproj, the three Dockerfiles,
deploy/scripts/migrate.ps1|sh, deploy/zitadel/provision.ps1,
deploy/up.ps1, deploy/README.md and docs/development.md. Also fixes a
stale Dockerfile comment pointing at a compose file that no longer
exists.

Verified with `dotnet build Musify.slnx` against the new layout.

backend/Hosts/Musify.Api/Program.cs

@@ -0,0 +1,48 @@
+using System.Text.Json.Serialization;
+using Musify.Infrastructure.Observability;
+using Scalar.AspNetCore;
+using Musify.Api.Endpoints;
+using Musify.Api.Extensions;
+
+var builder = WebApplication.CreateBuilder();
+
+builder.Services.ConfigureHttpJsonOptions(options =>
+    options.SerializerOptions.Converters.Add(new JsonStringEnumConverter()));
+
+const string devCorsPolicy = "dev-cors";
+
+builder.Services
+    .AddApplicationServices(builder.Configuration)
+    .AddObservability(builder.Configuration);
+
+builder.Services.AddProblemDetails();
+
+if (builder.Environment.IsDevelopment())
+{
+    builder.Services.AddCors(options => options.AddPolicy(devCorsPolicy, policy =>
+        policy.AllowAnyOrigin().AllowAnyHeader().AllowAnyMethod()));
+}
+
+var app = builder.Build();
+
+app.UseExceptionHandler();
+
+if (app.Environment.IsDevelopment())
+{
+    app.UseCors(devCorsPolicy);
+    app.MapOpenApi();
+    app.MapScalarApiReference();
+}
+
+app.UseAuthentication();
+app.UseAuthorization();
+
+app
+    .MapAlbumEndpoints()
+    .MapMixEndpoints()
+    .MapPlayListEndpoints()
+    .MapTrackEndpoints()
+    .MapUserEndpoints()
+    .MapYouTubeEndpoints();
+
+app.Run();