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/Shared/StorageKey.cs

@@ -0,0 +1,12 @@
+namespace Musify.Application.Shared
+{
+    public static class StorageKey
+    {
+        public static string Combine(params string[] segments)
+        {
+            return string.Join('/', segments
+                .Where(static s => !string.IsNullOrWhiteSpace(s))
+                .Select(static s => s.Trim().Trim('/', '\\')));
+        }
+    }
+}