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/Core/Musify.Application/Mixes/Responses/MixItemApplicationResponse.cs

@@ -0,0 +1,27 @@
+using Musify.Domain.Entities;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Application.Mixes.Responses
+{
+    public record MixItemApplicationResponse(
+        MixItemSource Source,
+        Guid? TrackId,
+        string? VideoId,
+        string Title,
+        string? Artist,
+        string? ThumbnailUrl,
+        double DurationSeconds,
+        bool IsExplicit)
+    {
+        public static MixItemApplicationResponse FromEntity(MixItem item) =>
+            new(
+                item.Source,
+                item.TrackId,
+                item.VideoId,
+                item.Title ?? string.Empty,
+                item.Artist,
+                item.ThumbnailUrl,
+                item.DurationSeconds,
+                item.IsExplicit);
+    }
+}