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/Tracks/Responses/TrackUploadUrlsResponse.cs

@@ -0,0 +1,16 @@
+namespace Musify.Application.Tracks.Responses
+{
+    public record TrackUploadUrlsResponse(
+        Guid PictureIntentId,
+        Guid AudioIntentId,
+        string Bucket,
+        string PictureKey,
+        string PictureName,
+        string PictureContentType,
+        string PictureUploadUrl,
+        string AudioKey,
+        string AudioName,
+        string AudioContentType,
+        string AudioUploadUrl,
+        int ExpiresInSeconds);
+}